Towbar wiring ( relays and dedicated wiring systems)

338 Likes Comments Comment
Like if this guide is helpful

This guide is to help you choose the correct wiring system or relay for your car. Car wiring systems have changed over recent years which has has made the wiring of a towbar much more complicated.

Wiring a towbar socket used to be fairly easy, you just mounted a socket on your towbar and ran a 7 core cable to the lights and connected it to the power supplies you needed . For information about how to wire a normal socket please see our other guide using the link below.

Trailer plug guide and wiring diagrams

This guide tells you which wire goes where and also state what type of plug / socket is best for the purpose you require.


Some years ago higher class vehicles began to be fitted with bulb failure monitoring systems. These systems are now being fitted to more everyday cars and vans.

Simple bulb failure monitoring systems (early systems)

These systems were the first ones. They were fitted in the 1990's to monitor the brake and tail lights you need to wire you trailer socket using  3 way bypasses.   

Modern wiring Systems

The 2 most commonally used wiring systems are multiplexed and Can-bus systems these are very complex systems. They only have one power supply to the light clusters.  The individual lights in the cluster are operated from the power supply by signal voltages sent from the controls.  The signal voltages are not sufficient to operate extra trailer lights. To overcome this problem you need to fit a 6 or 7 way relay, either of these will do the job but the 7 way relay also switches the reverse lights on a 's' type socket.

New advanced wiring systems.

Some new vehicles are now a little more complicated this does include some Multiplexed and can-bus systems, in addition to the above the system monitors for bulb failure and then will try to replace a faulty bulb by illuminating another bulb. For example if a side light fails it will light the brake light dimmer to substitute for the missing light.  This is done with a pulsing signal to the rear lights.  The pulses cause the relays to pulse and the lights on a trailer / caravan to chatter.  This is rectified by fitting a smart bypass relay, it has the means to smooth the the pulsing as it enters the relay. 


A smart bypass relay will work on all wiring systems so if you are unsure of the wiring system that your vehicle has this will do the work of them all with the exceptions lised below.

Some vehicles have slightly different wiring systems and will not work with the smart relays. please see the list below, this will be updated as we are informed of any other vehicles.

Jaguar                   S-Type              We can supply a vehicle specific relay

Iveco                    Daily                  2001 on we have not yet found a relay to suit.


Dedicated towbar wiring systems.

Due to the complexity of the new wiring systems the towbar suppliers have over the recent years started to supply dedicated wiring systems which are the best solution to the complex wiring systems on modern vehicles.  Most popular modern cars and vans now have a dedicated wiring system for a towbar available.  These are designed to fit a specific vehicle and the fitting of these can vary. 

Some of the more complex wiring systems in cars have a plug in the back of the vehicle into which you plug a dedicated wiring system for your socket. The wiring system just plugs in and the socket just fastens to your tow bar nice and simple.

Some cars do not have a plug and these dedicated systems are not as easy to fit as they still need wiring in to the vehicle but the wires are all the correct lengths and any relays etc are built in to the wiring. 

We have over 100 wiring systems available to us which we will be listing early in 2007. If you require one now we can supply a price upon request.

Please Note some cars do need Dealer configuration.  This means once you have wired the towbar up, the car needs to be plugged in to a computer to configuer it to run the extra socket(s).

Hope this has been of some help.

For information on products available please contact us

To find our other guides please click on the linkk below.

Our other guides

Explore more guides