Left rear stop light goes out when braking with lights on

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

What's a DHS ?
Stop Lamps

The STOP LP fuse in the rear fuse block supplies battery positive voltage to the normally open stop lamp switch. When the brake pedal is pressed, the stop lamp switch contacts close and battery positive voltage is supplied to the center high mounted stop lamp (CHMSL) and the turn signal/hazard flasher module stop lamp switch signal input. In response to this input the turn signal/hazard flasher module supplies battery positive voltage to the left and right LED tail lamp assemblies. The CHMSL and left tail lamp have ground at G402. The right tail lamp has ground at G401. The stop lamps illuminate when they receive both power and ground.
es battery positive voltage to the left and right LED tail lamp assemblies. The CHMSL and left tail lamp have ground at G402. The right tail lamp has ground at G401. The stop lamps illuminate when they receive both power and ground.

Power to the brake lights flows from Fuse 12 in the underhood fuse box to the brake light switch. Check for power at the switch on the orange wire. Power leaves the brake light switch on the white wire and heads rear fuse box under the left rear seat. Power goes to fuse 34 Vehicle stop fuse 15A. From there it goes to the left and right stop bulb connectors on the light blue wires. If you don't have power on the light blue wires, you've got a bad fuse or an open somwhere. Two fuses in the stop light cirucit.

Thing to check. cable broken or joint came apart, no power coming to lights, check both end of fuses with a trouble light, may look ok but be open circuit , check battery voltage at tail light and stop light switch , if none fault in cable to b+, if ok fault in cable to rear lights, check earth to rear lights.

GM had a lot of problems with the circuit boards that house the rear lights. My suggestion is to use a test light clip one end to the trucks body parts then remove brake light switch the orange wire should light the bulb on test light this is the power wire, the white wire is the stop light circuit with switch out this also should be lite up. Drop down turn signal switch connector the brake lamp wire goes up the turn signal switch and comes back down the yellow and green wires no power there it is a bad turn signal switch. If Ok Now remove one tail lamp assembly unplug harness and test for power if still have no power trace wiring back to fire wall and check at bulkhead connector. IF ok then circuit board is bad.

go to the brake pedal siwtch and check voltage there, the center light is not on the same circuit

you prob got a trailer plug prob bad wires if not tail lights run through turn signal switch tell fix ya bout me ok

OK ... being as you've ripped the entire interior out things might be easier to disconnect now .... I suspect the dash light dimmer assembly (I don't remember but this vehicle may even have a dimmer fuse??? check the owners manual ...) which goes to many lights and if damaged shows a dead short (has the radio ever been changed?) I would first disconnect the dimmer assembly (cluster lamps) (that will eliminate that circuit) replace the fuse and try turning the parking lamps on ... if the fuse blows again ... it's not the dimmer cicuit ... then ... You will measure a short when checking continuity between negative battery terminal and the light circuit as the filaments in the lamps will read continuity but it should show some resistance and not a dead short under normal conditions. I would then disconnect the harnesses going to the rear of the car located in the left kick panel and or the right kick panel .... the way you will solve it is to work one section at a time ... and a bunch of fuses ... trial and error then focus on the suspect section ... the thing that gets me is: you say the short goes away (on the meter) when the brakes are applied or left turn signal. So it might be in the rear harness ... I would also inspect the left rear light assembly and the harness to it (possibly a bad ground) good luck and let me know what you find

These are Dual Function Bulbs. They control the Parking Lights and the Brake Lights. This Bulb is Burnt out on the Brake Coil inside the Bulb. I have actually seen Bulbs that were Bad But you could not Tell by jast looking at them. It should be fine with a New Bulb. Please Rate My Response! Thanks!