ive bought a towbar but im worried about cockin the electrics up im from new tredegar in the ryhmney valley have you ever fitted a towbar if so can just wire the electrics off one back light ?
for single electrics you will need the left and right tail lights, left and right indicators, and a brake light, so you will need access to both lights.
You will also need the audible sounder wired into the indicator lights.
If your electrical car system is of the newer kind, Canbus, you will also need a bypass relay
If you don't need the bypass relay then yes, you can pick up the feed from the back of the tail lights in the car.
The wiring diagram usually comes with the buzzer, assuming you got the buzzer from new. The feed comes from each indicator, through the buzzer to the socket. You also need an earth for the buzzer if memory serves me correctly. Should be 5 wires to the buzzer if I recall correctly, 1 to each indicator, 1 earth and 1 each to the indicator pins on the socket.
They usually run up one side and then across the boot floor or inside the bumper, depending on your car.
I always run the 7 core cable to the rear light on the drivers side (as this side has the fog light) and connect into the brake light, right hand indicator, right hand side light and fog light here, I also earth the earth wire to the body nearby. Then run 2 separate wires to the left hand indicator and side light. However, there is no right and wrong way to route the wires, just go for the compromise that suites you between convenient and neat.
I assume you already have the wiring diagram, but they are easy to find with the help of the ever friendly Mr Google.
Would be useful to know exactly what vehicle you are fitting it to, as some (like Land Rovers) are simply a plug in job, whereas others need to go to a dealer to have the ECU reprogrammed (like newer VW's)
Rather than risk over loading any thing, I would consider a dedicated wiring loom. They are not particularly cheap unfortunately, but would still be cheaper than going to a tow bar fitter for wiring. Towsure list one for the Berlingo here These are simply plugged into the existing wiring loom so cut out the need for cutting any wires on your loom and therefore make the liklihood of any faults occuring much smaller. This would be the option that most towbar fitters would take anyway. I assume that a 2003/4 Berlingo has no towing stability programme and would not need a software update.