The leading problem with Trailer LIghting is… GROUNDS, Trailer and Vehicle Frame !
Find the ground, check it out for connection, a good, clean solid connection is needed and the wire should be sized to carry all current, so if you replace the ground/wire make sure you replace it with a good size wire. (Unless you are going to LEDs then a smaller wire will be needed).
If the grounding is good then check the sockets in the lights and the main connection.
Dirt, road grim and corrosion will decrease the effectiveness of the connections.
Checking with a multimeter will tell you there is some sort of connection but not how good it is, unless you are good at it and know how to check the resistance of the connection then you will not know for sure, it is easier to just take them apart, clean them up and be done with it.
My buddy had a trailer, supposedly was new but lights would work and then stop an another would give him trouble. He asked me for help and the first thing I did was check for the ground, WHAT GROUND, there wasn’t any. The trailer was actually attempting to ground through the trailer hitch; no wonder the lighting was intermittent. We put in a ground wire from the Trailer Connector on the Trailer Frame and all the light worked. Once the boat was in the water he decided he wanted to convert to LEDs, we rewired the whole thing and ran separate ground wires for each side of the trailer and each light was connected with two wire, power and ground, soldered and heat shrink with adhesive inside was used.
4 years later he has no problems and mentions it once in a while, he says, “Man I have no lighting problems in the trailer for a long time", (asking for trouble), and all I can say is "No $hit” !
So check the grounding on both sides of the trailer hitch, double check the grounds on the trailer, do the disconnect, clean and use a good quality waterproof dielectric paste on all connections.
Chasing a bad connection on a trailer is more time than hard work but a PITA to say the least, luck to ya !
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