Trailer Lights Nightmare

[Destroyer]

Originally Posted by spareparts

make sure you have a good ground from your truck to your trailer, grounding thru the trailer ball doesn't get it. take a jumper wire and ground the trailer to the chassis of the truck, see if that helps, if it does, start looking at the white wire

Totally agree...Always a good idea to look at the ground first. And you MUST wire the white wire directly to the trailer.. the trailer ball is a great way to get a poor ground.
But he said that the one side works great, so that doesn't sound like a bad ground at the hitch. It "might" be a bad ground at the trailer light itself, where the pigtail wire from the light gets put over the bolt, but I tend to doubt that also. I think Ferm might be on to something with the miswire internally, which is why I said to switch lights and see if that solves the problem. If it does then you know it's the light and not the wiring. If it doesn't then you know it's in the wiring somewhere.