I Hope This Helps
Sorry this is kind of late, but I just saw it & wanted to share what I've found, in case it helps you & others:
I have a 2000 Sierra Extended Cab & have, over the last two years, had several cabin leaks. Here are some I had to repair-
When I first bought the truck, the leak was what was described in a previous post- The door water deflector. Someone was in there & didn't put it back properly. (Hard to believe, but the black gooey stuff holding it on is OEM. I have a friend whose job is installing these at the factory.)
A few months after bought the truck, it leaked again. The grommet for the antenna cable was popped out and the cable took a downward turn, directing water into the cab. I re-seated the grommet & put a drip loop in the cable so water drips off. This was time consuming to diagnose & more so to access, so hopefully it is not your leak.
A few months later, one of my cabin pressure relief vents (behind the back seat) was loose. I removed both, cleaned them & re-installed with electrician's duct seal around them. A bunch of water came through that! I only found it because of the ice blob around it when I was cleaning up in winter.
A friend said he had a leak because the seal around the duct for fresh air went bad. He popped the metal cover over the duct (under the cowl) and re-sealed it. (My HHR has a similar setup, but the cover is plastic. It shrunk, let water in and I had a ductwork ice berg this winter. Killed the blower motor, too.)
Just to keep me in practice finding leaks- My sliding rear window's hard, plastic trim cracked in three places and leaked water down, behind the seat. There went 100.00- Out the window, so to speak.
My latest leak is (was?? as of this morning) the grommet for the wires going to the door. It was loose & rattly in the mounting hole. I unhooked the wires & removed the grommet. I heated up the tabs to bend them back out. I cleaned everything, applied electrician's duct seal putty & put it back. I was amazed how much water came through this hole.
One tiny leak I also had, on the other side, was the bracket for the door back-check was loose & some water came in around the screw. Found that one by mistake.
I hope you find the problem without too much trouble & your truck doesn't become a "Leak-Of-The-Month-Club" truck, like mine!
2000 Sierra 2500 4 WD SLT 3 Door 6.0
With Genuine Cardboard Pine Tree Air Freshener!