Strange Tail/Brake Light Problem

Hi,
I'm having a weird problem on my 01 Alero. (2 door, auto, 4cyl) The two brake lights in the tail lamps stay constantly on, but the third brake light will come on and off with the switch from the pedal. I've checked all the bulbs, fuses, relays, etc. and haven't been able to come up with anything.
If anybody can help or has some advice I'd appreciate it.
Thanks alot,
Jon

The signal from the brake switch goes to a module for the ABS braking system. If the third brake light works then the module is good. For the two rear brake lights the signal branches off and goes to the hazard switch on the dashboard before going back to the brake lights. So you may have a defective hazard switch with sticking contacts causing the light to stay on.