I would also suggest searching the forums here for either "CHMSL" or "CMSL".
Also, most people don't realize, but there is a separate switch that controls the Center High Mounted Stop Lamp or third brake light. It's down near the floor on the base of the steering column. My suggestion would be to purchase a service manual for your model year, and either test that switch, or just pick up a replacement and replace it. They're prone to burning out.