I have a 1994 GMC Sierra C1500 2WD with 4.3l w/automatic. It was a fleet vehicle. Plain Jane accept for High Output Heater, Limited Slip Diff and Engine Block Heater. Brake lights were working intermittently. Would come on when pedal as first depressed then go out after pedal traveled down a bit. I put in a new switch and all was well for 3 months. Then the problem came back again. I was ready to put a new switch in again when I took a close look at the wire connector on the harness. The end yellow lead was melted in the connector and the female plug for this wire was black. I got a boneyard replacement and wired it in. Problem solved.