Check to ensure that the bulb is not burned out, and that all of the wires are intact. Next, look on the frame behind the rear cylinder and you should see the brake light sending unit mounted there. If need be, follow the brake line from the rear master cylinder to the sending unit. There should be two wires going to the sending unit. One should be hot when you turn the key on, and the other should show hot when you apply pressure to the rear brake pedal. If this doesnt happen, your sending unit (pressure switch) is bad.