So I got pulled over yesterday because the 3rd brake light isn't working. I just checked the obvious, and there's no power at the fuse. I tried both sides of the fuse to ground, pushed the brake pedal until I heard a click and the other two taillights came on, and on one side of the fuse I got 0.5 V and on the other side nothing of course.

Is there a relay or something that feeds this fuse?

 

My guess is bad socket. Did you check voltage to ground at the socket?

 

model year, make, engine etc?

 

haha oops!

It's a 2006 Suburban LT 1500 with the 5.3 L Vortec

 

The power to the fuse comes from the brake pedal switch. You need to have someone press the pedal while you check the fuse for power. You can also turn on the hazard flasher then check for the voltage pulses at the fuse.

 

Are we overthinking this? Tap power at the side brake light, assuming it's isolated from the turn signal like a real car company, and send that to the top light.

If they are a combined one, then no dice. Also, I HATE combined turn/brake lights.

 

Youre certainly not the first to suggest that - no, they aren't combined and that's what I'll do if I can't figure it out. I like them separate but I wish the turn signals were red too.

Thanks for the information that it's fed directly from the brake pedal switch, I think that's probably where the issue is (it's got aftermarket remote start)

 

Is the power to the 3rd brake light fuse possibly the white wire off the brake pedal switch? It finally above freezing so I crawled under and looked and there's some weird vampire power tap coming from there for the remote start, just wondering if maybe they accidentally cut the wire.

 

The white wire feeds both fuses (VEH STOP & VEH CHMSL). Did you check for power at the 'VEH CHMSL' fuse( both sides)? If you have power at the fuse then the problem is with the center lamp/socket or the wiring harness to the center light.

 

I must have been checking it against a ground that had built up a coating over it...

I decided to just replace the light with a $20 special from eBay.

Yep, it was the brake light itself.