i had this exact problem. They said it leaks where the wiring harness AND where the parking brake cable enter the cab from the engine compartment. apparently this is a common problem on 07 and newer av's.
I too, had this EXACT same problem, in my 2005. So it isn't just the GMT900s... it's in the 800s as well.
The main wiring harness grommet can become unseated out of it's hole. It can be that way for a year or two or three without you noticing, but I caught mine relatively early... within the first few rains after taking delivery. Water slips inside the firewall and down under the carpet, saturating the jute padding underneath.
The main wiring harness grommet is about 4.5 inches in overall diameter. The best way to find out if it is unseated is at night. Put a droplight inside the cab at the driver's floorboard facing towards the brake pedal. Release the hood latch, and then look under the hood behind the fuse box, at the firewall, to the right of the master brake cylinder. If it's dark outside, and your grommet is unseated, then you will see light coming through your firewall that you shouldn't be seeing.
This light test is a lot easier to do to verify the problem before getting your hands dirty. To properly seat the main wire harness grommet might require removing the fender to firewall brace on the driver's side, and removing the cover to the underhood bussed electrical center (fuse box). I don't own a 900, so I'm not sure how it is configured, but I doubt it is too much different when it comes to a main wire harness about an inch and a half thick, going through a large grommet about 4.5 inches in diameter.
I also removed the threshold trim, the kicker panel trim, and elevated the carpet and the jute padding while placing small heater fans on the floor board to dry the area out completely.
Once my grommet was fully seated correctly, I did not experience any more leaks.
Prior to discovering the grommet (accidently, using the light test method prescribed above) I had done all other manner of things, the most inconvenient being blue taping the entire windshield surround every 3 days. I don't have a sunroof, so the windshield was the only source that came to mind.
I wish I had read this post and the post immediately above back when I found my leak on the driver's front floor. It seems crazy, but now, the first place I look for a leak is the firewall grommet.
Sure enough, I encountered another automobile... a Lincoln in fact, that had the same issue. Something to check always.