Wiring the white trailer socket for a tow car

[billbo45]

Just bought a Fiat 500 converted to a tow car. The idea is to tow it to wherever behind my Transit Motorhome. The tow car needs the secondary white tow socket usually used to power a fridge on a caravan when towing. I need the powere to supply the tow car's electronic braking system.
I have no issue with where the live wire goes on the white socket.
The question is, where do I connect the cable to for the power?

[MinorMatt]

There is no dedicated supply for a "caravan" socket.

Cables are often run along the underside of the vehicle and connected up in the engine bay.