You have back-up lights on a trailer? Sure? I've never heard of it. I'm pretty sure there is a ground wire on a 4-pin connector. It's the one that's exposed, while the other 3 are kinda protected by the design of the plug.
Corona


'98 TJ, 4 cyl., auto
31" AT KO's on rockcrawlers,
custom bumpers, Hellas
2" Teraflex lift, no top,
no doors, no problem.