The bed on my 1960 f600 was condemned by me. Bought good used bed. Took my time on old bed in unhooking miles of "rigged" wiring. Labeled things as I pulled. when was done eliminating all unneeded wiring I ended up with what believe is the factory wires for tail lights. Each side had 2 prong plug with corresponding plugs on tail lights. I have one mystery wire that 12v hot when lights are on. This wire slipped by during undoing of many splices and butt connections. FYI truck has dual filament bulb serving as running/ turning/ brake lights.Any suggestions?

 

License plate light?

 

There are two possible tail lamps, round or the shield type. Both incorporate the license plate lamp on the left side.