Trailer light pigtail - LCD's - FYI
As an FYI:
I was having problems with my trailer lights. Whenever the headlights or parking lights were on the turn/brake lights would also energize. If the headlights were off, the turn, brake lights worked fine. In other words, on the 4 prong plug ( ground, running, L Turn, R Turn) both L and R Turns would show 15V when the headlights turned on.
I took it in to the dealer who replaced and checked all the relays and fuses.
It turned out to be the aftermarket LED's I was using in my trucks rear tail lights. When he replaced them with regular bulbs the problem vanished.
I would not have believed it if I did not see it myself.
Apparently the trucks computers were sensing too low a resistance in the lights and defaulting to an all on situation.
Luckily I have a very cool dealership ( Mod friendly too! - I do not have to return to stock for maintenance like this) and he did not charge me for the tech work.
Moral of the story: If you are having weird issues with your trailer lights and have aftermarket LED's installed in your truck, try replacing them with factory incandescence and see if that solves the problem.