I don't have all trucks memorized so you'll have to tell me, ... Does yours use separate bulbs in back, one for the signal and another one for the brake light? Most manufacturers have gone to that. It greatly simplifies the turn signal switch but it makes it miserable for connecting the two independent circuits to one circuit on a trailer.
My guess is you have two separate bulbs because you said the turn signals work. If that is correct, the signal switch is not part of the system so we can forget that. Switches won't cause a blowing fuse anyway because there isn't anything in them going to ground, therefore, no matter what would become shorted, it would just make the wrong lights turn on or at the wrong time.
Do my light bulb trick so you don't have to keep wasting fuses. Once the brake pedal is held down and the test bulb is bright, move harnesses around, particularly in back, and remove the rear lamp assemblies to see if disturbing any of those things makes the bulb go dim. When it does, you'll also see the brake lights turn on dimly.
If you have a four-pin trailer connector from U-Haul with three red LEDs on it, those often shorted leading to blowing fuses.
Monday, January 6th, 2020 AT 2:21 PM