BG is most likely correct, especially if the Hazard Light Knob has broken off. GM used (may still use) a combination Hazard/TurnSignal switch, thru which the Main Brake lights are routed. Check to see if the Cyclops (upper) Brake Light is working. If it is and the main (tail) Brake Lights are out. The upper light does not route via the Combination Switch. Autozone carries the Combination Switch for around $50.00.
1995 Chevy Lumina Owner.
July 2009