I see a fellow GJ'r is also an RV'r :)
Both wires of any light can be hooked into the harness if the trailer has been wired with a dedicated ground circuit all the way through to the connector plug. Many better quality lights are made with a ground wire, they don't all just rely on a mounting screw to ground through. This is better, IMO, than relying on a ground jumper from the plug and then bonded to the trailer frame and then all lights also bonded to the frame -- this has more weather-affected connections and more potential for a corrosion to interrupt the grounding....of course, a dedicated ground wire run throughout needs to have good connections to be any better....
There are modular harness systems designed for big truck type trailers (tractor trailers). They are very nice and weatherproof, but are very expensive. Too expensive for the private user to spend that kinda money on one little landscape / utility trailer / camper. Most people just get a roll of the 4-wire trailer cable and go to work hooking it up. I have used some SO cord (rubbery type extension cord wiring) as it is a little better protected - no wrap / loom needed in a harsher environment, just string it from connection to connection and secure it with wire ties.
