1995 Chevy Tahoe water intrusion

I have a leak coming into my passenger floor through my fresh air intake flap area at my blower motor under the dash. I have taken off the rain/air grill on the hood, the wheel and wheel well, and the only thing I could find was a long rubber gasket thing running vertical down the side of the firewall had come loose. But there doesn't seem to be any fresh air intake holes along that area. Everything seems to be in place, but everytime I put water down the hole, it leaks into my floorboard. Where is the fresh air intake hole, and how can water be getting in there?
Thanks,
Adam