Switch your two trailer lights around. Plug the left in the right, and the right in the left and see if the problem follows.The best invention for trailer wire problem finding, is to get the truck side version of the plug, and put ring terminals on the ground, running lights, brakes/turn signals, and backup lights. Then you can use a spare battery to run which ever circuit you are testing. You know it works from that far back and you just follow it till it fails.Im usually swearing at a bad ground.