Both the brake light wiring and the turn signal wiring go up the steering column to the switch and can be effected by using the tilt , how and where the headlights figure in , unless the dimmer switch is on the turn signal arm is a bit of a mystery .
Wiring repair 99% of the time spent is finding the exact cause of the problem.
JMHO: I'd be pulling the turn signal switch & wiring, out of the steering column. And examining all the wiring in the trailer connector . These would be common places where the three wiring systems run together , and I would hope they were the first places checked.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.