David and Tim,
British vehicles, inc Jeeps in Brit service, were fitted with a a small lighting socket, similar to a bulb holder, there was a two way switch fitted adjacent and a convoy lamp, the switch would cancel the rear lamp and light the convoy, and vice versa, but maintaining feed to the socket, the trailer also had a rear lamp and convoy lamp along with a two way switch, and if trailer had a tow hook on the back, then another lighting cable socket.
There is a recent thread on G503 forum on this same subject.