The brake lights on my 99 Blazer are not working the third brake light is. I had a brake controller and 7 pin plug installed about a week ago. My gut is telling me its the multifunction switch. Went down that road with my old 01 Blazer. Im just wondering if the plug could be the cause.

 

I just had this exact problem. There have been a few posts relating to this and a GM recall to address this as well. Try wiggling the hazard light switch on the steering column or pushing it on and off several times. Once my hazard lights worked OK, then my brake lights worked. I guess you need to exercise the hazard light switch periodically.

 

I bought a new multifunction switch cuz i called the dealership and was told of no such recall. SO i get it all apart and i notice that the knuckle head that installed the brake controller when he tapped into the wire for the brake controller he crimped the wire and acutally cut it. So i fixed that and put it all back together and returned the switch. so all is good for now