G'day M8!

I was a USN Avionics tech for 20 years and a month, so I know a bit about the weird things electricity will do. It was my job day after day to sort out what went wrong right down to individual transistors, resistors, and the like inside of very strange devices.

THIS to me, sounds like a wire was tapped into during the trailer light install that should have been left alone.

I also rather HIGHLY suspect that it isn't anything to DO WITH the headlights at all - the same circuit controls every one of your side and top marker lights. From what you have said, it is THEM that are messing things up.

You've got an incorrect wire cross somewhere in there... Most likely between a side marker or tail light and a turn and brake wire

It DON'T have to be a wrong installation either, it could be a pair of wires that have been cut into or shorted together some way, but the money is on a bad setup.

I would cut the entire trailer harness out (or at least the wire for the brakes, and do it over), and reinstall it carefully and by the numbers from scratch were I you.

~Hope something in there works to the good!

~Dutch J. from the USA