2003 Chevy Silverado 2500HD Lt Brake Lite on 5er not working

I have had a TT with this truck and all the lights worked fine - but I upgraded to a 5er and the left brake lite on the 5er doesn't work.

The right brake lite works fine.

FWIW, the clearance, brake, left turn, right turn and hazard lites all work.

I connected my brother in laws chevy and everything worked fine. So, it's the truck. After chasing down the wiring, I got to the fuse block inside the truck (Chevy's have two one under the hood, one inside the truck). I found the fuse that is for the left Stop/Turn Signal and put a volt meter on it. When the Hazard or the left turn, it shows +12Volts, but when I put the brake on - nothing. 0 volts.

When I repeat this on the Right fuse, I get +12volts when the brake is on, when the hazard is on or if the left turn.

So the problem is in the truck. Since the Chevy's have separate Turn Signal's and Brake lights. Otherwise I'd just wire it from the back end and be done with it.

What I need to know is how the two circuits are combined in the truck? I can buy a unit that does this in the auto parts store - but chevy has already done this because it's part of the two package when I bought the truck.

Any thoughts?

Thanks!

josh