Mike, Having owned several trailers over the years and helping neighbors get their lights working, I have to say THE NUMBER ONE PROBLEM Ive found is A BAD GROUND,,,,, ,,,, Things that can help are to get a known good made gorund at the trucks witing connector. I often welded a small stainless bolt to the frame near the connector and used that for the ground wire into the trucks connector, anddddd d do the same on the trailer. Next common problem I found is those cheap press on crimp wire connectors. I got to where I cut them alllll l out and re stripped and re spliced the wires using twist on wire nuts then siliconed or taped over to keep water out. Another issue may be the differences in the 3 versus 4 wire systems and the use of the 3rd extra center brake light circuit n gettin that mixed up between truck and trailer. They make solid state 4 to 3 wire converters ya know. This is where the truck has seperate turn signals (4 wire system) instead of the regular 3 wire dual element (1157) tail and brake/turn bulb system used on the trailer. On those typical 3 wire system trailers the green is the right turn n stop,,,,, yellow is left turn n stop,,,,, ,brown is tail lights,,,,, ,,white is ground. Id, CHECK THE GROUNDS ON BOTH TRUCK N TRAILER CONNECTORS,,,,, ,,,,, , insure good wire splices,,,,, ,,,,, ,,insure youre NOT mixing a 4 and 3 wire system (unless a 4 to 3 wire converter is used),,,,, ,,,,, insure youre not mixing up any 3rd wire brake light circuit on the trucks wiring receptacle,,,,, ,,,INSURE THE TRUCK N TRAILER PLUG N RECEPTACLE IS WIRED CORRECT!!!!! !!!! John T
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