check the floorboards and seams to see if there are any holes rusted through. I had an old Mercury Topaz a few years ago that had a few holes rusted through under the carpet and anytime I was driving in the rain the carpet would be soaked wet. I also had a large hole rusted through in behind the wheelwell area. I patched it up using some roofing tar, and it actually held up quite well, it's a cheap, easy fix.
