GMC Repair: 2001 gmc envoy brake lights, turn signal switch, rear lamps

Question
I have notice that the brake lights does not come on when brake is applied but there is taillights when headlights are turned on. The third light is not working also, I replaced the brake switch but still no lights when brake are applied. The fuses are all good. What could be wrong?
Thx...

Answer
Hi
I would double check and see if power is getting to the STOPLP fuse.
If so check the CHMSL relay located in the underhood fuse box.
Since the "3rd" brake lamp is inoperative it HAS to be a problem on the blue wire from the CHMSL relay.
It MAY be the underhood fuse box itself.... the power is distributed to the fuses and relays internal in the box.
I also see they ALL share ground G450 which is the ground for all the rear lamps.
It is located in the rear on the drivers side near the taillamp harnesses.
Other than these items, I would REALLY need the vehicle to check it out.
There is an OUTSIDE chance it COULD be the Multifunction turn signal  switch. IF by some chance the 3rd brake light is just burned out, I would then HIGHLY suspect the turn signal switch.
Check for power on the blue circuit to the 3rd light.
I hope this helps.
Follow post back, if needed.
Pawl