GMC Repair: brake lights, 30 amp fuse, brake light switch

Question
I HAVE A 2000 GMC 3500 P/U.
WHEN THE VEHICLE IS COOL ALL THE BRAKE LIGHTS WORK.
(REAR AND CAB). WHEN THE VEHICLE WARMS UP THE CAB BRAKE LIGHT STILL FUNCTIONS BUT THE REAR LIGHTS WILL NOT OPERATE. THE SIGNAL AND FLASHERS STILL WORK BUT NOT THE BRAKE LIGHTS. I HAVE CHANGED THE 30 AMP FUSE UNDER THE HOOD AND CHANGED THE BRAKE LIGHT RELAY.  

Answer
Did you test the brake light SWITCH at the brake pedal when this fault occurs ?.

What mechanical work or ANY kind of work has been performed on the vehicle recently ?. As most problems that occur after a recent repair was made can be traced back to the area of where the repair was made as a wire and or ground connection may be loose.  Look around in the area where work was performed.


Also check around the rear of the vehicle as there is a BLACK ground wire that is attached to the frame that goes to BOTH rear tail ligh assemblies.

Do the EMERGENCY FLASHERS and turn signals work during this time of your fault ?.


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