My 3rd Brake Light stopped working sometimes or was very dim. The regular brake lights worked fine. Took it to the dealer and after about 40 minutes of 2 techs. troubleshooting this is what they found. Both circuits go through the brake light switch. The regular brake lights then go to the BCM which in turn sends a signal to put 12 volts to the regular brake lights when the pedal is depressed. The 3rd brake light gets it's 12 volt power directly from the switch. They could jump the 3rd brake light to a 12V source and it would work fine. They found there was only 5 volts coming to the harness in the tailgate. I am not sure if there are different contacts in the brake light switch for the 2 circuits or it just takes less voltage to tell the BCM to turn the regular brake lights to come on or it may just need a make or break signal to the BCM. I don't know. They didn't have a brake light switch in stock so they cleaned the burned and pitted part on the contacts and it is working fine now. The new switch will be in in the morning. Just thought this might be some helpful information to know. The brake light switch would been one of the last places I would have looked because the regular brake lights were working fine.

 

they might have some "bulb out" circuitry in the reg brake lines, don't know why not the high center. I saw that somewhere b4 but don't remember where now.they show export wiring direct to the bulbs and the high center,as I see it right now.

the 5v sounds like a logic signal to turn the circuitry on. oops, but why in the back?

looks like the sw has just 2 contacts huh?

 

If this is an H3 under the top half of the plastic façade, the one around the window, there is a plug near the hinge side of the rear door that is there for ease of assembly on the assembly line. This plug gives trouble. The rear window defrost wires also go through this white connector. Wiggle the wires while the center light is plugged in, your buddy has his foot on the brake, and see if the brilliance changes if it does the contacts in the plug are loose. Cut one color of wire at a time and splice until all four wires (two for the light and two for the defroster strip, have been cut off the plug and the problem will go away. I strongly recommend a very good crimp splice or soldered and insulate with shrink tubing.
Any one having third brake light problems? According to the local Chevy dealer there are none to be had. I have successfully repaired these lights and made changes so that the original problem will not occur again. I charge for successful repairs. If I cannot repair (fixing involves coat hangers and duct tape) the board I charge nothing but shipping and handling.

 

I have a problem with my 3rd brake light......only half of it lights up.

 

i looked at mine last year to see if i could beat the system with a few light bulbs,,no luck. seems to me there is a couple groups of VERY bright leds and with luck a good solder technician might figger it out and fix it!

the leds gotta be brite to get thru the plastic junk that covers it up and bulbs arent that brite.
i didnt want to scrape the stuff off my glass and deface it so i gave up.

 

Well I have to get mine replaced before May or I'll fail yearly inspection (even if only half of it lights up). Any Idea what they cost?

 

$?? but very pricey.

 

I have the same problem. The wiring diagram for the truck shows two groups of leds in parallel inside of the 3rd brake lamp module. My guess is that it's a burned circuit trace, bad solder joint, or a cooked current limiting resistor. It should be an easy fix if I can take the unit apart.

Does anyone know if the circuit board is accessible?

 

yeah, some work to open up, very abrite leds,hurt your eyes!!

 

Again, I have had success repairing these lights. According to Hendrix Chevrolet there are no new ones available.