If all else fails buy a tester for a few dollars. Get the one that shows lft trn, rt turn, tl lghts, and brk lghts. If everything checks out ok on your tow vehicle, then buy a new boat trailer wiring kit w/submersible lights included. That is what I ended up doing to fix my father's wiring problem when replacing bulbs and checking grounds didn't cure his trailer light problems. Just a thought because it only takes a few dollars and 1/2 hour to 1 hour to fix it and know it's all right. Good luck.