I have a very odd problem im trying to diagnose but am having a hell of a time trying to wrap my head around. My GF's 07 cruiser just started having this issue where the drivers side brake light will not work and actually turns off when pressing down the brake pedal when the headlights are turned on. After much testing this is what iv done and tested so far but with no effect on the brake light.

- Replaced brake light switch- Replaced turn signal switch- Reset TIPM by completely removing the unit and draining residual power in the car then putting everything back hours later.- Swapped bulbs around to see if it was the bulb, it was not.
- Removed tailight assembly and checked all wires connected to the bulb socket, even rewired the wire in question (orange wire) to assure strong connection.
- Replaced the #3 red 10amp brake switch feed fuse in the TIP

What's odd about this is the brake light functions as it should when the lights are turned off but once you turn the lights over to anything other than off the brake light on the driver side will turn off when braking where the passenger side and top 3rd brake light all illuminate as they should when braking. To add even more confusing into this, there is an aftermarket light bar that is connected to this troublesome taillight and i initially thought this was the cause of the problem however i have since disconnected it and the problem still persists. Another observation iv made is that the light bar actually doesn't react the brake at all anymore when connected, however if i disconnect the troublesome taillight completely and only connect the light bar then all of a sudden the light bar reacts to brake but again only when the lights are off with the lights on it does not illuminate for brake. If both the troubled taillight and light bar are connected the light bar doesn't illuminate to Brake at all and the taillight only illuminates to brake when lights are off. This makes me think it's a power draw problem not getting enough power when the lights are on. Ill post some pics when i can, any suggestions or thoughts appreciated.

 

Check the ground wire(s). If the brake light goes out when the headlights come on, it's because the ground wire can't pass enough current to operate both filaments in the bulb. Trace the black wire from the socket to where it grounds to the body. Loosen the screw and wiggle the connector to break up any corrosion. Re-tighten the screw. Should do it.