A lot of trailer wiring arrangements rely on frame-ground and hitch-ball contact to get a ground. I prefer to run a ground wire around to each light
and ensure it gets into the ground connector of the plug system. On trailers with house batteries, I also connect the frame ground to the battery
(-) terminal. In fact, I recently ran a stout ground wire from my truck battery
back to the rear frame *and* also connected it to the trailer ground connection.
When you stop and consider that it's possible to have all your trailer lights
, your reefer and your trailer brakes
full on all at the same time, plus maybe some other parasitic loads, while your truck is trying to recharge the trailer battery
, you realize that you want a grounding arrangement that can carry the returns for all these loads simultaneously without failure or degradation of current -- that's a potential for a *lot* of amps headed for the trailer with only one ground lead to serve them all, and it might not be a good time to expect the other stuf to give up their share to the brakes
Pete and Rats who don't have any grounding wires at all...