Towbar Electrics Guide

What kind of electrics does my vehicle need?

There are generally four varieties of connectors that you can purchase in the United Kingdom and three different varieties of electrical kit. Therefore it is a matter of personal choice and preferences. Usually the connector that matches the trailer’s or caravan’s light board connector is fitted.

Which type of kit can I fit my vehicle with?

An after-market dedicated kit, original equipment dedicated kit or universal kit might be available for your vehicle. There are several factors that will influence your decision.

Is it best to get a Universal electric kit?

These kits are often appropriate for use with a majority vehicle types. The drawback is that they are not designed with a specific vehicle in mind, and research and specialist knowledge may be required to fit these. These kits come in two varieties; the first of which is the basic kit. This is powered off the lighting circuit of the vehicle. The second is the bypass kit. The vehicle’s lighting circuits and a dedicated power supply serve as signals to switch between relays in this instance. Universal kits can be the fastest method and are often the cheapest, but the user benefits and functionality are limited. Certain vehicle types may be incompatible with this kit, such as vehicles that have more complicated electrical systems.

Is an after-market dedicated electric kit suitable for my vehicle?

After-market dedicated kits have the advantage of being vehicle specific. These kits come with all the perks of original equipment kits, but at a lower price. If you have a new vehicle model you may find that availability is an issue. In fact, when compared to the original equipment kits they are often 30-50% more cost effective. Dedicated kits will not invalidate the vehicle’s warranty because they have been designed to fit a certain vehicle.

Is an original equipment kit suitable for my vehicle?

The selling points of an original equipment kit are reliability and greater user assurance. The installation of this type of kit is covered under the manufacturer’s warranty and it has been designed alongside the vehicle at the point of manufacture. Cost can be an issue and the kit might only be readily available if it is purchased with a specific towbar. Problems can potentially arise with the 12S caravan supply; sometimes the pin functions do not conform to United Kingdom requirements and alterations are not facilitated. Witter towbars recently tested in excess of 80 vehicles. 93% of those that had towbars fitted and supplied by the main dealer came with electric kits that were not compliant with full UK specification 12S function.

What types of connector are available for my vehicle?

Your vehicle can be fitted with West, 13 pin, single and double connectors. There are several factors that may influence your decision.

What are the advantages of a 7 pin single electric connector?

At present this connector is the UK’s most cost effective and popular connector. It is compliant with all normal UK towing requirements. This connector is very versatile as it is suitable for use with caravans, cycle carriers, horseboxes, plant trailers, garden trailers and lighting boards. Almost anything that can be connected to your towbar can be used.

What are the advantages of a 12N 12S, 7 pin double electric connector?

Double connectors consist of the normal (single) connector but they also come with an additional supplementary connector. The supplementary connector is suitable for use with tipper trailers, caravans and horse boxes with permanent and switched power supplies, reverse light facilities and additional earth connections. If you want to power a refrigerator or charge your battery while you are on the move, a 12S socket will be required. The main drawback of double connectors is that the two connectors can be cumbersome and can appear ungainly, especially when used in conjunction with a towbar which is detachable.

What are the advantages of fitting an ISO 13-pin connector?

A single connector is roughly the same size as 12N connectors. It combines the functions of both the 12S and 12N connectors. It has the advantage of a waterproof connection, and the twist lock system is a unique feature of this product that allows for extremely reliable and secure contact. These sockets are very neat and if you have a towbar with a swing away style socket plate this product is ideal. Although using 12N/12S connectors costs less, all UK caravans that were manufactured post 2008 come with a 13 pin connector.

What are the advantages of a West 13-pin connector?

This socket is compatible with 12N plugs and it also looks just like a 12N socket. It can be used with lighting boards and garden trailers. 12N/12S connectors are suitable for use with your caravan’s West plug if it has 6 extra pins for supplying the 12S function and the standard 12S configuration. NEVER USE WEST SOCKETS WITH METALLIC TRAILER PLUGS.

Which connector is the best, the West 13 pin, the ISO 13 pin or the 7 pin?

While the 7 pin connector is the most widely available and cheapest, it is not always reliable and it can be cumbersome. 13 pin connectors are secure, reliable and do not have the disadvantage of quickly becoming obsolete. However, they can prove to be more expensive and harder to source than other types of connectors. West 13 connectors have the advantages of convenience and they are good value, but they are the hardest to source. It is important to bear in mind that all UK caravans that were manufactured after 2008 come with a 13 pin connector.

What are the different kinds of adapters?

If you need to interchange between UK 7 pins and ISO 13 pin systems you will need a connection adapter:

  • Not all twin adapters come with full 12S function conversion
  • As all UK caravans that were manufactured after 2008 come with a 13 pin connector, adapters are a major component
  • The four kind of adapter are MP600B – 12N to 13 pin, MP602 – 12N/12S, MP601B – 13 pin to 12N and MP603 – 13 pin to 12N/12S

My vehicle has a 13 pin socket and my trailer has 2 X 7 pin plugs. What type of adapter should I get?

You will need the MP603 adapter.

My vehicle has a 13 pin socket and my caravan has a 7 pin plug what type of adaptor should I get?

You will need the MP601B adapter.

My vehicle has 2 X 7 pin sockets and my trailer has a 13 pin plug. What type of adapter should I get?

You will need the MP602 adapter.

My vehicle has a 7 pin socket and my caravan has a 13 pin plug. What type of adapter should I get?

You will need the MP600B adapter.

Are all 13 pin kits identical?

You will find three main varieties of 13 pin kits:

Vehicle Specific 12N function only:

  • This is basically a standard vehicle specific kit that comes with an attached 13 pin socket
  • All normal power for legal light functions is supplied to the trailer or caravan by this kit
  • This includes side lights, fog lights, brake lights, turn signals or indicators
  • The kit does not come with a caravan/trailer supply feed
  • 1 – 7 pins are contained in the socket
  • However, pin 8 for reverse is sometimes included
  • This is now the most common vehicle specific kit and it is standard

Vehicle Specific 13-pin with 12S preparation:

  • This is basically a standard vehicle specific kit that comes with an attached 13 pin socket
  • All legal light functions are facilitated by this kit and 12S function can easily be added
  • All 13 of the wires and pins are usually contained within the socket
  • Pins 1 – 8 are fully functional
  • In order for the 12S extension kit to function as a 13 pin kit, a 12S extension is required which incurs an additional expense
  • These extension kits include wire for the power supply that must be routed to the main battery, a split charge relay socket for connection to a socket that is pre-prepared and all of the required fuses

Vehicle Specific 13-pin with full function 12N/12S:

  • This kit comes with full 12N and 12S function
  • All of the normal lighting functions are performed by this kit
  • The kit’s supplementary power supply facilitates additional functions for the caravan or trailer
  • No further kits are necessary
  • The kit comes with all connection details
  • There is the added advantage of this being compatible with the newer European and UK manufactured caravans

What is a 12N connector?

There will be a socket on a car and this provides normal power to the caravan or trailer for the sidelights, fog lights, brake lights and indicators. This is supplied by the motor vehicle’s 12N connector when the trailer or caravan is plugged into it.

What does a 12N connector look like?

You can usually expect to find a black socket cover. Seven pins numbered from 1 to 7, are housed within the socket cover. The plug is usually black and there is a slot that can be found opposite the socket’s cover hinge. The seventh pin is the female and also the central contact.

How should the 7 pins be connected?

12N connectors need to be wired correctly in order to be connected, and this must be in accordance with DN/ISO 1724. This is illustrated clearly in the diagram below. A diagram is referred to here but is missing from the site.

What cable do I need to use?

There are seven wires contained within the outer sheath of the 12N seven core cable. These seven wires should commonly be in the region of 0.65mm², but the earth wire may be larger, around 1.0mm². Refer to the diagram below. A diagram is referred to here but is missing from the site.

Are there standard colours for the cables?

No. You may find that wire colours vary according to the vehicle manufacturer; this is usually the case with original equipment and vehicle specific kits.

What is a 12S connector?

There will be a socket on a car that provides supplementary power to the trailer or caravan for the fridge, battery charger, reversing lights and interior lights. This is supplied by the motor vehicle’s 12S connector when the trailer or caravan is plugged into it.

What does a 12S connector look like?

You will usually find a white or grey socket cover. Seven pins, numbered from 1 to 7, are located within the 12S socket cover. The plug is often grey or white and a slot can be found opposite the socket’s cover hinge. Pin number 7 is the central contact and the male.

How should the 7 pins be connected?

12S connectors must be wired correctly and this must be in accordance with ISO 3732. You will need to refer to the diagram in the user manual. You will need to ensure that wires 3, 5 and 7 are independently earthed to the car when the 12S connector is wired. Where the vehicle’s split charge relay allows, you should connect the blue wire that comes from pin number 2 to the 2nd switched output terminal. This will facilitate compatibility for caravans manufactured before and after 1998.

What cable do I need to use?

There are seven wires contained within the outer sheath of the 12S seven core cable. These seven wires should commonly be in the region of 0.5mm², but the white earth wire may be larger in size, 2.5mm². The diagram below helps illustrate this. Refers to missing Diagram

Can you explain what a split charge relay is?

This relay simply splits the vehicles power supply to the 12S connector.

Why would you need the relay?

This prevents your vehicle’s battery from becoming flat. The relay works by isolating the caravan from your car. Pin 4 provides a permanent supply of power solely to the interior lights of the caravan when your engine is off.

How does the split charge relay work when I start my engine?

The relay powers pins number 4 and 6 for the fridge. If it is wired for use with pre 1998 caravans, you can also use pin number 2 for the charge circuit if you require more power for a larger refrigerator in a post 1998 caravan.

What about pin 4?

Modern caravans have their own relay switch. When the vehicle’s engine is running the power is switched from the interior lights to allow the caravan’s own battery to charge.

Is this facility available for my caravan?

If your caravan was manufactured after 1998 it will probably have this facility. Pre 1998 caravans will use pin number 2 as the battery charge supply.

What is a 13 pin connector?

13 pin connectors are also known as continental connectors. These combine the 12N and 12S functions within a single connector and can be found on some caravans and vehicles.

What does a 13 pin connector look like?

There is usually a black socket. Thirteen pins, numbered from 1 to 13 are located within the socket cover. A key feature of this connector is the twist lock mechanism that helps to secure the plug and socket.

How should the 13 pins be connected?

13 pin connectors must be wired correctly in accordance with European and UK specifications. Usually pins numbered 1 to 8 will use a 1.5mm² wire, but pins 9 to 13 will use larger 2.5mm² wires.

Could a 12N or 12S socket or plug be used with 13 pin?

No. The 13 pin is incompatible with the 12N and 12S. If you do need to interchange between these systems adaptors are available.

Do some systems combine the 12N and 12S functions within one socket?

Although the West socket appears to be identical to a standard 12N 7 pin socket it actually comes with extra pins. These are located between the 7 pin configuration which enables the use of both 7 pin and 12N plugs for light boards and trailers or the caravan specific West 13 pin plug.

Is there a way to tell if the towing electrics are functioning properly whilst driving?

A tell tale is supplied with your vehicle’s electrical kit. This will alert the driver if there is an indicator failure.

What is a tell tale?

The function of the tell tale device enables a driver to see whether the trailer’s indicators are functioning correctly. There may be a dash mounted warning light or an audible bleeper operating in unison with the indicators and, in the event of an indicator failure, this will stop. Some types of tell tales only alert the driver in the event of an indicator failure, but some tell tales signal an indicator failure by causing the car’s indicators to flash at a rapid rate.

How often should caravan/trailer lights be checked?

It is very important to check full light function each time you connect.

What is a fog lamp ‘cut out’ for?

The vehicle’s fog lamp needs to be disabled during towing in order to reduce the reflected glare from the trailer/caravan.

Does every car come with a fog cut function?

You will need to check whether a wiring kit has this function as standard. If not, you can request it at the time of installation.

Could I fit the electric kit to my own vehicle?

You will find complete instructions in all modern electric kits. If in doubt it is important that you consult an approved stockist who will be able to carry out the installation on your behalf.

What can happen if a bulb fails on a vehicle?

Usually the trailer or caravan lights will still be operational. The vehicle may have CAN bus wiring, multiplex or bulb failure monitoring, so you could be notified in the event of a bulb failure. This would be dependent on the model. Any failed bulb should be replaced as soon as is convenient.

What is a dedicated kit?

A dedicated kit, as the name suggests, is a kit that has plugs, connectors, a wiring harness, relays and a control unit that have been designed specifically for your vehicle.

What is included in a dedicated kit?

Most dedicated kits typically consist of a quality 7 or 13 pin socket, comprehensive vehicle specific diagrams and instructions, communication and power cables and plug-in connectors that come with the correct length of loom. Additional fuses, grommets and cable ties complete this kit and aid in the completion of the installation process.

Is a dedicated kit superior to a universal kit?

The answer here is a definitive yes. Dedicated kits are designed and built to work in tandem with the electrical system of your vehicle. They may communicate with a vehicle’s other control units to alter other vehicle settings as required. This will deactivate the rear fog lamps and parking aids on the vehicle and will activate the dash trailer indicator warning light.

Does my vehicle need a dedicated kit?

Dedicated kits are likely to become compulsory for vehicles because they offer the most reliable and secure connection. This enables your vehicle’s safety functions to be fully operational. << I could not find anything online to back up this change in dedicated kits being made compulsory in the future, but this is what it says on the Witter site and on the others.

Is it hard to fit a dedicated kit?

Although the kits come with comprehensive instructions more skill is required when it comes to installing this kit when compared to others. This is due to the kit being specifically designed to fit a certain vehicle. Generally the installation is not difficult and it takes around an hour to fit. A recode may also be required, on some models, by the main agent. The user benefits and enhanced safety make the extra cost worthwhile.

What benefits do you get with a dedicated wiring kit?

Dedicated kits can have the capacity to modify (model dependent) vehicle systems: anti-lock braking, traction control and stability control program systems. This is definitely advantageous when it comes to towing.

Are the benefits noticeable?

Although many of the benefits are not immediately apparent there are a few obvious benefits. Automatic deactivation of original parking sensors, automatic fog light cut out, and trailer light failure warning are among some of the benefits that people notice first. The primary benefit is reliability.

Why doesn’t it beep when I indicate?

Some dedicated wiring kits come with a C2 module and these kits do not operate a towing dash warning light or beep while you indicate. The driver is alerted to an indicator failure by a visual warning.

What does C2 (control) mean?

>Legislation in Austria, Germany and the UK dictates that a driver must be able to visually or audibly check the functioning of the indicators on the trailer. The C2 control is standard in kits for certain vehicles that have sophisticated electronics.

What if my trailer indicator bulb fails?

If the left or right bulb fails, the vehicle’s dash light and indicators flash rapidly to alert the driver. Certain kits use a text display as an alert and this will be seen on the dash panel. There are also kits that simply beep in the event of an indicator failure.

Does a car always need a dealer recode when fitted with a dedicated kit?

A recode will not be required after most kits are installed. This is because if installed correctly, a kit will handle all of the full light functions. A full recode may be required if you want to benefit from all of the additional features such as enhanced safety and various user functions. The recode details will be included in the kit.

Why are the colours of the wires going to the socket different to UK standard colours?

The wires in the dedicated kit may differ in colour to standard UK colours if the kit conforms to ISO1724. This is shown below. Site says refer to diagram, but there is no diagram.

There is a yellow wire and it doesn’t connect to anything, what is it for?

The majority of kits connect to the 12S socket’s reverse pin via an output if twin electrics are a requirement. The yellow wire usually serves this purpose.

There are extra wires on my socket that go to a switch, what are they for?

A majority of dedicated kits come with two or three additional wires that go to the socket’s micro switch. The purpose of this is to allow the trailer control unit to verify whether a trailer is connected.

What does the micro switch do?

Generally speaking the micro switch disconnects the vehicle’s reversing sensors and rear fog lights for towing.

Can I use the dedicated kit that comes with a 13 pin socket if a caravan has two 7 pin plugs?

Yes, but you will need an adapter. These adapters convert from 13 pin sockets to 12N/12S 7 pin sockets used for powering twin electrics. They can also adapt a 13 pin socket to a single 12N 7 pin socket for powering normal light functions.

Can I use the dedicated kit that comes with a 7 pin socket if a caravan has a 13 pin plug?

There are adapters that convert 12N/12S 7 pin sockets to the 12N/12S functions in a 13 pin twin electric sockets, or a 12N 7 pin sockets to a 12N 13 pin normal light function sockets.

What are the main benefits of having a dedicated kit?

Here are the key benefits of dedicated wiring kits:
  • Connection to the vehicle’s sockets is quick and easy
  • The connections are extremely reliable and secure and dedicated kits are built to last
  • All necessary modules and components are included in the kit
  • A dedicated kit will not damage the vehicle’s wiring
  • The kit comes with a comprehensive user manual complete with clear and logical assembly instructions
  • All components are good quality and the equipment is often original

What is multiplex wiring?

Multiplex refers to a communication and multi-function power cable that attaches to each control unit. Control units distribute power to chosen devices. This is also referred to as power-line communication or PLC.

What does CAN mean?

Controller Area Network or CAN refers to the array of control units that respond and communicate with one another on the CAN bus network.

What does ‘bus’ mean?

Bus is the name given to a network a line that attaches directly to all of the control units. Each unit will only respond to its intended signals because each unit has a unique identity.

What does it look like?

This depends on the manufacturer, but a CAN bus usually consists of two wires that are twisted together. These are CAN low and CAN high.

What does CAN high mean?

The ABS control systems, safety features and engine control units operate via the high speed signal network. Hence the name CAN high.

What does CAN low mean?

Electric windows, seat electronics and lighting systems operate via the low speed signal network. Hence the name CAN low.

How does CAN work?

A CAN bus network can be likened to a postal system. A data signal is a letter with a postcode on it and a control unit functions as the mail sorter that has an allocated postcode and address.

Let’s use the example of brake lights. When the brake pedal is pressed the control unit registers this and letter saying ‘operate brake lights’ is sent. The letter has the postcode for the ‘rear light control unit’. The letter gets sent on the CAN bus and the postcode is read as it passes through the control units. If the postcode on the letter doesn’t match the control units’ postcode it is simply passed along until it reaches the control unit with the corresponding address.

In this case the control unit with the corresponding address is the rear light control unit. When this letter reaches the rear light control unit it is opened and read. Now the rear light control unit knows to operate the brake lights. This all happens in a fraction of a second; almost instantly.

When was CAN first used?

The Robert Bosch Corp created the CAN protocol in 1984. The first instance of its application was in 1992 and the vehicle in question was a Mercedes Benz.

Why use CAN?

These days most vehicles use these systems because it is lighter, cheaper and more reliable. There is also the added advantage of it helping with diagnostics.

Why is CAN cheaper?

Regardless of specifications only one loom is required per model and fewer connections and less wire speeds up the installation and design.

Why does the weight matter?

A traditional car loom weighs roughly 40% more than the multiplex/CAN bus loom. When cars are lighter the results are less emissions, greater economy and improved performance.

Why does this improve reliability?

Poor connections are often the cause behind many electrical faults. When you have fewer connections and less wire this improves reliability. You can download post production modifications and improvements into registered vehicles.

How does this make diagnosis easier?

Faults are registered and stored. A technician can retrieve this information if they need to rectify a fault. In-built diagnosis allows for some functions to carry on operating in a limited capacity if a fault does occur, rather than failing completely.

What does it look like?

Having the correct flow diagram or CDF will help you to identify CAN bus and multiplex. Can bus wires are rather thin and they are twisted together.

Why have the wires been twisted?

This reduces interference from other power sources.

Am I able to test it?

The signals can only be seen with LED test light or a digital storage oscilloscope. These signals can only be read if you have the correct software on your laptop or commercial CAN scanner. Multiplex appears to be 12 volts, but you may disturb the signals if you try to connect to it. You can try testing a CAN bus using a multi meter and this will show up as either 4 to 5.5 volts or about 1.5 volts.

Should I avoid it while testing wires?

It is advisable to test as close as you can to the light units. Never test half way along a loom because you can’t see the starting and finishing points. You must use a no load tester; either an LED test light or a multi meter.

How can I tell if my car has these systems?

The switching is electronic in cars that have these systems, so you will find fewer fuses and relays. The control unit shuts off power if there is a fault until it can be rectified. Vehicles with a multifunction screen, controller dial or a comprehensive Driver Information System use CAN bus systems.

Which cars have CAN bus?

Cars that have been manufactured since 1992 come with some form of these systems.

Is it possible for me to fit towing electrics to a car with multiplex or CAN bus?

The multiplex and CAN bus system can be operated if you use a dedicated kit. This also allows for the automatic activation of many safety features.

What is the bypass relay?

A bypass relay works off the rear lights of a vehicle in order to switch a relay. This enables the trailer’s corresponding functions via the relay switches and a permanent separate power supply.

Is a bypass relay suitable for me?

If a bypass relay is fitted, the activation of some safety features including the Trailer Stability Program will be negatively affected. Your vehicle will not recognise the fact that it is towing. It is advisable to contact your dealer or check the vehicle’s handbook.

What is a dedicated kit?

As the name suggests, this kit includes relays, control unit and vehicle specific wiring harness that specifically operates and fits your vehicle.

Is it better to have a dedicated wiring kit?

It is better to have a dedicated kit because it has been specifically designed and built to work in tandem with the electrical systems of your vehicle. Some kits can communicate with other control units so as to alter vehicle settings and this can deactivate the rear fog lamps, parking aids and activate the dash trailer warning light.

Is it difficult to fit dedicated wiring kits?

Dedicated kits are generally easy to fit. Some technical knowledge is required because they have been designed to fit to a specific vehicle, so some will be easier to install than others. It generally takes around an hour and you may require a recode from the main agent depending on the model. The enhanced user benefits and safety outweigh the extra time and cost that is involved.

What benefits would I get from a dedicated kit?

A dedicated kit can alter the anti-lock control system, suspension settings, stability control, traction control and other (model dependant) vehicle systems. It can also enable the trailer bulb failure warning and automatic fog cut out. Original equipment parking sensors can also be deactivated.

Does my vehicle have to have a dedicated wiring kit?

It is likely that dedicated kits will become mandatory for all vehicles at some point in the future. This is because they enable the entire vehicle’s towing safety features to be utilised, as well as being very reliable and safe.


This morning I found the tell tale permanently on in my motorcaravan. The van has not been used for about 10 days. All was okay when it was last parked.Any ideas? Hi John, without looking at the vehicle it is hard to diagnose, it sounds like the towing socket or related wiring could have some water ingress/damage or corrosion causing a short circuit. Regards Mark
what will i needie kie / relay for 08 zafira to wire to a cycle board

Hi if you are fitting universal wiring on the vehicle you will need a bypass relay any zafira after 2005 requires a bypass relay regards aaron
is uneversal kit ok for bmw e39 2001

Hi Derek universal wiring will work perfectly on that vehicle however you would need to fit a bypass relay aswell alternatively we can offer a dedicated wiring kit for around £132.60 if this is any interest to you
regards Aaron
will I need,a bypass relay
on corsa 2010

Hi Karl starting from 2006 the vauxhall corsa does require a bypass relay regards aaron
Hi,I have a 2013 focus titanic hatch with self park and camera aids Will I require dedicated wiring and download? thanks

Hi, we do recommend dedicated wiring kits for new cars under warranty, for a focus titanuim 2013 the kit would need coding and will then deactivate the parking sensors. Thanks
Have a Zafira 2004 model, can i just attach standard loom with scotchloks and buzzer? No relays, etc required?? Thanks

Hi the vauxhall zafira year from 1999 all the way upto 2005 doesn't require a bypass so yes you can just fit a universal wiring kit with scotch locks and a buzzer
I see some kits listed supporting Trailer Prep or non-Trailer Prep. How can I tell if my car has or doesn't have the trailer prep?

Hi your vehicle will have the plugs in the boot if it has trailer preproration i would ring my local dealer and double check with them just to be safe
I have a 2007 Hyundai Santa Fe fitted with two 7 pin sockets, can I use one of these to connect a trickle charger to maintain the car battery during periods of non use?
Is a Universal Wiring Kit with Bypass Relay for 7 pin electrics (12N) suitable to fit to my 2008 Fiat Ducato and if so do I need specialist knowledge? Also will my vehicle need a dealer recode after this Wiring Kit is fitted?
Hi Colin Yes the 7 pin kit with bypass realy is suitable for your vehicle, no dealer download required, the 7 pin pre wired socket connects into the bypass relay and the relay wiring conects into the vehicles wiring, we do have a technical help line if you get into difficulties.
Regards Brian
Do I need a By pass relay on a 2005 Fiesta and if so where is the best place to pickup the live feed, at the battery or the feed into the fuse box (glove compartment).
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