Towbar electrics P38

Am about to install the white socket on the P38 towbar. Does anyone know whether there is a prewired connection tucked away inside somewhere? I've had a look behind the rear left light cover and there is a suspicious looking ununsed plug there. Might that be it?? The towbar is already wired with the black socket so towing is ok for trailer lights and things just now but its when the caravan is hitched I need the white one too.

The plug you refer to has wires for 12v+ Ignition, Negative & (I think-reversing lights). The negative wire is too small to use for powereing the battery charge & fridge-you will need to get a good separate earth from a self tapper into the inner wing or the bolt holding the sub woofer (If fitted) to the body. You can use the 12v+ ignition to power the relay coil for your grey caravan socket battery charge & fridge pins. Suggest you run a 4mm sq. single cable via a 20amp fuse from the positive battery terminal to feed the normally open contacts on the relay for the battery charge & fridge. I ran mine through the bulkhead (R.H. side),kick panel, lower carpet skirt, underneath the rear seat to the l.h. rear light cluster area.
Hope this helps.

Thanks for response, I'll get the multimeter out tomorrow and check which wire is which. I didn't just want to branch in to that plug in case something went wrong - you know how temperamenal a P38 elecrics can be!