Trailer Reverse & Cargo Lamps for a specific application:
http://www.pirate4x4.com/forum/electrical-wiring/2283114-trailer-wiring-diode.html
The center wire in the diagram is the trailer battery charge circuit in a standard 7-terminal trailer connector.

This circuit works with or without the trailer battery, but obviously the lights won't work unless the trailer is connected to SOME battery some way or another.

Owner caption July 2016 · Photo 0 of 5000