Been there, done it. Problem is no brake/TS on the left side, rear. I have the running light but no brake/TS light. Ground is fine, bulb is OK. I pulled the plug at the firewall and had nothing there so I figured the problem had to be in the switch housing. Took it all apart and the contacts are all wore down. Went to NAPA to get a replacement and the replacement doen't mate up to the cowling. I did plug the two connectors up under the dash and the problem went away, so I know that's what's wrong. Appartently, the common replacement is for Chevy, not GMC. I was thinking that if I had "the blue wire is...." info, I could wire in a Mickey TS unit.
