Help with wiring on a trailer

[jjirons69]

Me and my BIL had a similar issue a few years ago with a boat trailer. We spent the majority of an afternoon about to pull our hair out. As your buddy is doing, we took back both lights and the wiring harness (4-pin). New ones went in within 10 minutes with no problem. We never found out what the real issue was. It will drive you crazy!