[Cryptichound]

I had the same issue with my 2004 impala, where water was coming in thru the cabin air filter. The panel under the wipers is split into 2 halves, one on the drivers and one on the passenger's side. The passenger side panel is removable to access the cabin air filter (its held on by 2 plastic expanding rivets, and a single plastic "nut"). There should be a strip that's attached to the bottom of the windshield that prevents water from getting to the filter that can be dislodged.

http://filterlocation.com/wp-conten...n-and-replacement-Chevrolet-Impala-step-2.jpg

hope this helps