Third brake light (center) is not working how to repair

can't seem to find the problem ???? That's understandable , if you don't know the proper way to test automotive electrical problems . Viewing wiring diagrams an using a DVOM - digital volt ohmmeter to test the electrical circuit is the correct way to diagnose . The third brake light works or not ? Is this what your saying ? But left an right sides don't ? Looking at the wiring diagrams I see the brake lights ,right an left or turned on by the Integrated power module (computer) Check for codes would be the first step. Not a code reader , no auto party stores , you need to have a qualified repair shop with a professional scan tool check it .

Check engine light on ? Have it checked for DTC'S - diagnostic trouble codes . All problems maybe linked together . If you don't want to take to a qualified repair shop , auto part stores will check codes for free ,most will ! i hear advance charges $20 now. Third brake light may just be a bulb or the Stop lamp relay in the under hood fuse box.
Stop Lamps
The body control module (BCM) is responsible for controlling the stop lamps. When the driver presses the brake pedal, the BCM will receive a signal from the brake pedal position sensor. The BCM then supplies battery positive voltage to both left and right stop lamp assemblies and to the center high mounted stop lamp (CHMSL). The stop lamps are grounded at G401 and G403. The CHMSL is grounded at G302.

As in the previous comments the brake switch wire goes to the turn signal switch and the third brake light. Since the 3rd brake light is working, the problem is in the turn signal switch. When the switch is centered, both brake lights have 12v to them. When you make a left or right turn, the switch has to switch connections between the turn signal blinker and the brake light circuit for the appropriate illumination of the proper light sequence. Buy a new turn signal switch and temporary plug it in. You will see the proper operation.

Usually when the third brake light works but the regular lights don't the problem is in the turn signal switch.
I would check for voltage there.

The first thing to check is the brake switch(es) that are on the brake pedal lever, and the circuit for the third brake light is seperate from the normal brake lights.

If the brake light switch(es) and wiring do check out alright then the problem will most likely be in the turn-signal switch, and the brake lights are ran through the turn-signal switch.

Hello. The symptoms you are describing are likely due to a poor ground. Temporarily install a new ground wire by securing a piece of copper wire to the frame with a self tapping screw. Use this ground wire along with the hot wire to see if each of these lights works. If it does, trace the ground wire back to the frame, unscrew it and clean up the metal with emery cloth so you reestablish a good ground.

The right brake light has lost its ground. It's a fairly well known problem on the '99 to '04 Grands.

have you checked the earthing continuity-if it's this other bulbs may also partly light when brakes used
otherwise-
Check continuity of live from center to lh & rh lights and/or is there a live at those lights

i have replaced many of these. it is the multifunction switch under steering wheel cover.the third brake light is on different circuit .so replace muti switch and you will fix your problem thanks please rate this answer

perhaps there is a relay in the engine compartment.