I went to change the brake light since my blinkers and brake lights are not working. My rear running lights do work. I pulled off the tail light to find this and I really am hoping someone can explain it to me.

 

Interesting. Looks like a previous owner had a failure of the turn/brake light circuit, and "fixed" it by adding a light bulb/socket. I'd follow the wiring and see where it is tied in to the stock harness.

 

Yeah thats what I was thinking too but I wanted a quick sanity check. I just got home and I'm about to try and track the wiring down. Whats crazy is it looks professionally done. I just can't figure out what would have made someone "fix" whatever light problem they had like this

 

I followed the wiring harness to the front of the vehicle and it seems like it is connected to the front trailer hook up. I guess thats where the added light is getting either a braking or turning signal. However it seems like the OEM wiring harness (for the normal braking, running, and signaling light just above the added light) is untouched. So I'm unsure as to why the OEM light is not working. Fuses seem to be fine and I know the bulb is fine because the running light works. For now my only options seems to be to pry at this adhesive they used to keep this new bulb in and pop that one out and replace it.

 

Check the trailer wire harness under the rear bumper and frame for wear. It is a know issue that effects the operation of the taillights in different ways depending on which wire is frayed.

 

So this is gonna be a bit of an embarrassing update. I couldn't find any wear on the wire harnesses so I took my hummer to the closest dealership to go ahead and take a look. Right away the service department guy that I talked to made an assumption that the previous owner towed this hummer around often and installed the extra bulbs for the trailer hook up on the front. That prevents the needs for having any power on in the car for the lights to work when towing the hummer (thats what he told me). Then he asked if I had replaced the brake light bulbs yet... which I hadn't... because I thought they were working since the bulbs worked fine for the running lights. As it turns out, there are 2 different filaments inside the bulbs for the tail lights. One filament stays on constantly for the running lights. The other turns on for brakes and signaling. Both filaments dont always burn out. So only my braking and signaling filament inside the bulbs had burnt out. I replaced the bulbs, everything works fine. Thanks for the help everybody. At least I know how these bulbs work now haha.

 

Too bad you didn't read my post in the thread "Tail Light Won't Come On"
https://www.hummerforums.com/forum/h...nt-come-43822/

 

Ugh! That would have been perfect. I kept searching for brake and signaling problems so I never found it

 

That was too obvious, OP must have had 1 burned at a previous date, and didn't notice, until both burned.