If you want to check the brake light switch for problems, just remove both wires and short them together. The lights should come on. If they do, the switch is the problem. Otherwise, just start tracing voltage with a simple illuminated test light until you find where the voltage loss is happening. The brake signal goes up into the steering column to the turn signal switch, then back to the bed, so there's a possibility the turn switch is part of the problem. The brake signal and the turn signal voltage comes from different places, so it's possible for brakes to work but not turn, or vice versa.
Jerry
