Wiring for 12N towbar / trailer socket

Fitting a towbar is generally an easy and uncomplicated job, but wiring it often puts people off doing it themselves. My quick guide below shows how easy it is and in general can be applied to most cars without problems.

I have based these instructions on the Vauxhall cars I’ve fitted towbars to:

Cavalier (mk3, mk2 is similar)
Astra Mk2, Mk3 and Mk4 (Nova and Corsa are similar)
VectraB (prefacelift and facelift)
Omega (Carlton and Senator similar)

You will need:

screwdrivers
wire cutters
pliers
drill and drill bits up to about 12mm
a hand reamer (to make holes larger if needed)
round metal file
rubber grommet
insulation tape (pvc self adhesive tape)
either a soldering iron and solder, or 7 scotch locks and a pair of mole grips

Wiring diagram:

12N towbar wiring

Wiring the 7-pin 12N socket for trailers, caravans and lighting boards:

Remove the trim panel that runs along the bottom of the boot, where the tailgate closes onto, this may reveal a small wiring plug close to the centre if your car has already got pre-wiring. If not, don’t worry, you’ll still need this panel off to conceal your cables later.
Remove the plastic panels (or open the carpet flaps) to get access to the backs of the rear lights.
Look under the car or behind the rear bumper for a suitable place to drill a hole to take the cable through into the car near the centre of the towbar, check both sides to make sure that when a cable is inserted it won’t be in the way or damaged when trim is refitted or luggage is put into the boot.
Drill the hole and open it out to the size needed to allow the rubber grommet to fit, remove any sharp burrs and swarf and then fit the grommet.
Pass the cable up into the boot space through your new grommet and make sure it’s neither too tight nor slack enough to wave around. If it isn’t secure under the car, use cable ties to hold it to something that doesn’t move, such as the tow bar. Don’t be tempted to route the cable next to the exhaust as it won’t survive for long in the heat.
With the cable fed through into the boot, decide where the connections will be made. If you found the pre-wiring connector, then it is easiest to make most of the connections to that. If not, then you will need to remove the outer insulation from a long length of the new cable so that you can take individual cores up to the rear lights.
Make connections as per the diagram below, the colour codes refer to the towbar socket cable and the task is to identify which wires they connect to on the car. There are several ways to do this and Vauxhall use the same wiring colours in pretty much all cars, but I always trace them through manually to make sure that there have been no changes – it’s a safe method and I’ve summarised it below.
When you come to the left and right indicators, fit an inline warning buzzer (available from Halfords etc) to comply with the law.
Connections can be soldered (if you have the gear) or made with scotch lock connectors if you prefer. Soldered connections are far superior as they don’t generally corrode or become loose later if done properly. Once each connection is made, cover with PVC insulation tape to be sure that no exposed copper can make contact with anything else.
Before refitting carpets and trim, check that all connections look sound and either connect up your trailer or use a multimeter to test.
Refit the trim when you’re happy that everything works correctly and have one last check that the wiring is still lined up correctly through the grommet as it enters the car and is still secure.
Have a beer and congratulate yourself on some hard earned cash that you saved and can spend on more bits…

Tracing connections:

This sounds fiddly but is actually pretty easy. I’ll explain the method and if you understand the principle, you can apply it to all sorts of other situations. Lights are at the easy end of the spectrum. You will need a multimeter (about a fiver from Maplin) or a low wattage 12V car bulb with wires soldered to it. Connect the black probe of the multimeter to the brown wire in the light cluster and use the red probe to measure Volts on each wire / track.

Remove the lamp holder cluster from both rear lights. For each connection, identify the bulb that is used and follow the metal track to the wiring connector to the vehicle loom. Make a note of the colour of the wire and it’s purpose. Repeat for all 7 wires and you have the connection list for your trailer socket.

Note that some cars use dual filament bulbs and so you need to be careful not to confuse the sidelights with brake and fog. It’s easy to use a multimeter to see which track is live when that bulb is lit.

Buzzers:

By law, trailer wiring is required to include a buzzer that sounds when the vehicle indicators are working, so as to produce a ‘beep beep beep beep’ sound inside the car when the trailer is connected. The wiring I’ve described above will be fully functional in terms of operating the lights correctly even without the buzzer, but as they are cheap and the wiring is trivial, do connect one.

The connections for that are simple and basically the buzzer is connected between the trailer socket and the car’s rear lights so that the power to the trailer goes through it. Insuructions are always included with these items so I’ve not covered them here. Get in touch if you have problems.

Bypass relays:

Some cars are fitted with bulb test / failure warning systems (called check control on Vauxhalls) which can be fooled by your trailer connections. The Vauxhalls I’ve seen don’t suffer, but the E Class Mercedes I used to have needed a bypass relay to work. It’s a simple thing – 6 wires in, 6 wires out, a 12V permanent live and an earth connection. All it does is draw a tiny current from each lamp circuit to switch a relay that drives the trailer socket.

And finally…

I hope these instructions are helpful, I’m always happy to go back and amend them if they’re not clear or if you find a mistake. Equally I realise I’ve not included photos in this guide and can go out and take some if they will help somebody. Just get in touch via the ‘contact’ link on the left. 🙂