I know this post is several years old, but I've been chasing this same leak on my 06 Suburban Z71 for a year. The rear carpet by the rear A/C unit was constantly getting soaked when it rained. I finally decided a couple of weeks ago to track it down once and for all. I removed all of the rear interior including the carpet, the panels, and the rear headliner. I bought a smoke generator to see if I could find where it was leaking, but that was a no go. I found water leaking at the bolts for the roof rack, so I resealed the rook rack - no more leaks at the bolts but still a leak on the floor. I replaced the body pressure relief valves - no leaks. I unmounted the rear A/C and checked the condensation drain tube - not the cause. I ran new window sealant around the windows - no leaks. After pulling what little hair I have left out of my head, I took one last shot at it. I had the wife shoot a hose at the passenger rear roof/window area while I sat in the back of the vehicle. I can now say definitively that I have tracked down the cause. The passenger rear drip rail lost its weather-stripping a while back. Now, water has found it's way under the weld and into the cabin. I watched it pour right into the cabin from the roof weld. I'm going to seal the weld up and re-test it in a day or two once the roof fully dries.
In the attached photos you can see the weld in the passenger side drip rail in the first two photos and you can see the weather-stripping still intact on the driver side.