That's the switch on the pressure differential valve which turns on the BRAKE (P) (!) light dash when one circuit of the hydraulic system loses pressure. It doesn't have anything to do with the brake lights. Your problem could be in a few different places.. Do your hazards/turn signals work? do you have cruise? If so, did it stop working at the same time as the lights? Someone else had this problem not too long ago and it was the hazard switch stuckin between on and off.
Matt {peace}
Matt {peace}