Re: lights not working on my trailer???
You are going to have to get a continuity light or a volt, Ohm meter to check out your system. First see that the vehicle plug gives the proper voltage to the running lights on yhour trailer. Standard code lists the brown wire as the running lights. No voltage or too low equals no lights. Correct it.
If you have the proper voltage atr the vehicle plug, trace the wires out to the lights on the rear of the trailer. Improper voltage there is a no go also. Correct the wires up to the lights if needed.
The last aspect would be corrosion on the light bulbs or light fixtures. Polish, check, replace if needed. Normally if you have stop lights on each side the fixtures are OK, but not always.