I have running lights back up lights everything but brake lights and my third light doesn't work.
Is there an aftermarket trailer harness in there? If so, remove it, make sure the factory wiring is all hooked back up, and go from there. Third brake light on a GM will be controlled by a separate switch on the brake pedal, as it's supposed to be redundant in case of primary brake light failure, not tied together. Theoretically you could wire it with the primary, just make sure your brake lights work like was done for the ~80 years before the retarded third brake light. Those lights are for stupid people that don't maintain their cars, not for responsible people.
Because the older model didn't have high mount brake lamp. There was a design change in a few things from those years. You shouldnt have used that old of wiring harness
