If your third brake light works, but the corners don't, you most likely have a defective MFS (turn signal switch). It is also possible that there is an open circuit between the brake switch and the MFS. Try reseating the big plug on the MFS and see if you get lucky.
The brake light signal goes from the pedal switch to both the third brakelight and the MFS. Inside the MFS, the turn signals are "added" to allow the brake lamp to flash when signalling is required. Since the signals wirk, it is safe to assume that the lamps and the associated wiring is functional.
Steve
