Rear brake lights stopped working

Light bulbs require power and ground in order to work. The ground circuit for the left rear combination lamp is shared between the turn lamp, stop lamp and tail lamp. Because the tail light still works in that specific combination lamp, we know that the shared/common ground for that assembly is OK. The power circuit for both stop lamps (left and right) have the same origin; voltage flows from the STOP LP FUSE (15A) through the DBC RELAY (when the stop lamp switch is pressed) and then to each stop lamp via their respective circuit branches. Because the right stop lamp works, we know that the shared/common fuse, relay and switch are all OK. I'd suspect an open in the left stop lamp power supply branch and would recommending checking for voltage on the green & black wire at the left rear combination lamp assembly to confirm. Regarding the left lamp on the hatch, the wiring diagram indicates these are not stop lamps rather tail lamps only. I'd recommend installing a tail lamp bulb instead of a brake light bulb into that assembly as a first step. You can read more here: https://www.yourmechanic.com/article/how-to-replace-your-car-fuse-box-by-jessica-howe

You mean the brake lights don"t work ? Do you have power in and out of the brake switch ?

I can almost 100% guaranty the turn signal switch is the problem. Power for the brake lights starts at the brake switch. power then splits from there and the third brake light gets power directly from the brake switch. Since the turn signal and the brake lights are on the same bulb power for the brake lights is routed through the turn signal switch. the switch depending on what position it is in will allow brake power or turn signal power to there respective left or right brake/turn bulbs.

If you hold your foot on the brake pedal and wiggle the turn signal and or play with the hazard switch the brake lights will almost always intermittently start to work. this will confirm a bad turn signal switch which is pretty common on GM vehicle with this style setup.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

sounds like you have a bad turn signal switch.

If the third brake light is working than we know the fuse is good and the switch is good. The third brake light bypasses the turn signal switch. If the rear turn signals are working than we also know that the wiring from the switch to the rear bulbs is also good. The brake lights and the turn signals are on the same bulb so the turn signal switch is what determines if the bulb gets power from the brake light switch or from the turn signal flasher. If the switch is bad usually the first thing that stops working is the brake lights.

Typically you can wiggle, jiggle, pull, push on the switch lever with you foot on the brake and get the brake lights to work. Sometimes turning the four way on and off several times will also get the brake lights to work. If this is the case than a new turn signal switch will solve you problem.

If you have any questions let me know!

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

the switch controls both front and rear signal lights.the wires just T off in the harness to go both ways.you don`t say but are the rear bulbs ok.there is 2 filaments in the bulbs one for tail lights and one for signals/brake lights.so the brake lights work if so then the bulbs are good.if not then the bulbs may be the problem.it could also be a wire feed/ground problem that you will have to trace from the sockets back through the system with a test light checking for power and ground.

This is a common GM specific problem. Your turn signal switch contains an elaborate networking device that has to cancel brakelights while running a turn light. Often times the ground within the turn signal switch is lost causing your situation. After investigating such other possibilities as fuses and loose grounds it may become necessary to replace the turn signal switch. Thank you, Dana

Are the rear bulbs OK?
Is the ground good at the rear?are the sockets good?
Are there tail lights and a license plate light?then the ground is OK.
All the things that you say do not work go through the signal light switch....it may need replacing.
First make sure the bulbs and fuses are OK and power is getting to the signal light switch and to the brake light switch.
Nice car!

does the third brake light work when braking ? the bar on the rear window? if not replace brake light switch on brake pedel

Do you have a trailer plug? Look for corrosion around taillight plug under truck in the back, PULL IT APART to see if there is any corrosion. If you find none, than take the taillight housing out and run a external wire from a clean spot on frame to the ground on housing, it could be a ground issue. Did you test light the break light switch power in power out?

You should check the fuses. There may be a separate fuse from the brake lights and the interior brake light.