It is a known problem, if you would have visited your local GM garage they could have told you about the problem and the solution.
When the truck was assembled, there is a very good chance that one of the seams in the body was not glued / calked properly.
The spot where it is leaking can be found online - lot's of other forums such as the Avalanche Fan Club - we all had the same problem as you.
I did the repair with a dab of windshield urethane, before that, you could not drive it in the wintertime with the heater on the floor, because the carpet was always wet and the blower - hot air would cause the windshield to fog up.
Just another one of GM's screw ups!
The leak is usually on the firewall - behind the carpet in the drivers foot area, behind the brake pedal, or near where the cable for the E Brake is located.
You can find it with a garden hose and some compressed air!