Trailer wiring ????

I have a 4 prong flat configuration for my trailer and when I put a test light to the tailights I get a dim signal on the test light (both brown wires). Everything else works turn signals and brakes are fine. The tail lights on the truck also dim as soon as I plug the trailer in. Any thoughts on the fix???
Thanks in advance.
John

High resistance in tail light circuit on trailer. Check for bad butt connectors or scotch locks.