[Longhorn Man]

Anyone who knows how to do basic wireing could hook up a third brake light without that stupid controller.

The brake light rout of power goes from the fues box, to the switch on the pedal, to the turn signal (very common for the hit and miss operation you describe) and then it goes out to the tail lights. No controller, no relay, no stupid gizmos... nothing.

With an assistant watching the tail lights, sit in the cab with your foot on the brakes... wiggle the turn signal lever very slightly up and down. If you acuate the signal with the ignition in the off position, then that side light will just turn off. Try lightly pushing it forward and back too.
If all is good so far, place the signal in one of the on positions (as in turn signal on) and try the wiggle up down forward back... then the other direction, same test. your assistant is looking for both lights to go off.
If it passes this test, then suspect a bad ground on the bed... esp problematic if your bed has a wooden floor, or, if the bed has been removed any time recently.