My car was part wired with a heavy lead for all the towing circuits at the factory, from the small/main engine compartment fuse panel and is a permanent live source. I then ran than to the rear 2x 4mm, to supply a voltage sensing relay and to feed the fancy extra towing module for road lighting/ lamp failure unit. That then connects to the light switch module at the front, using data on a single thin wire, to provide the LSM with data so it knows if any trailer lamps have failed. The car knows when it is towing, so disables the car brake lights, the reversing sensors, somehow adjusts the car for better towing ability and rather than the legally required bleeper when the indicators are working OK on the trailer, it only lets you know when they are not working. The reverse sensor are also supposed to take into account the tow ball stuck out the back, when not attached to trailer.
Earlier cars I have wired, I used the ignition/charge warning light circuit, to trigger a normal relay to engage, when the warning light went off.
