There's no dedicated "answer man" on this site, so it may take a while for you to get input. If you need one right away, go to the dealer.
Do you have an aftermarket alarm? If not, scroll through the steering wheel HUD menu
and check to make sure you have "horn" selected on alarm type. I'm not sure what exactly it says, but it's there.
On my old truck there was a plunger that when it was in the out position (or door open) the lights would come on. It got fouled, and no matter what position it was in the lights would not come on. I replaced it. I don't know for sure, but there's no plunger assembly that I can find right now because I'm parked tight in the garage and can't fully open the doors. Maybe you'll find one.
http://www.racepages.com/parts/door_lock_switch/tahoe.html
Try the steering wheel HUD for the horn... and try opening a different door and see if the courtesy lights come on. If they do, then you'll narrow it down to one specific door. If they don't then you may have an electrical wire loose or a short.
Hope this helps...