Why would the floor on a car's passenger side get wet when it rains?

Best bets: leaky windshield or passenger side window molding.

you might want to take the cover off that goes across the lower front part of the windshield usually this includes removing the wipers look for a obvious hole or crack in the joints if none are there put a garden hose up in this area and turn water on then get inside car on passenger side and look to see where water is coming from,

As a certified auto mechanic, I am often asked this very question. I usually recommend closing the passenger side door when leaving the car for the night. Other solutions are possible of course (some people prefer to carry a large roll of cling-film in the glove compartment and use it to seal the passenger-side doorway after parking), but I believe this is the most efficient and time-effective solution. It also makes use of a little-know design feature of the car, its hinged and fully-locking doors. How's that for serendipity?!?

I own a 1996 Grand Am GT and it leaked on the passenger side. (Apparently, these cars are notorious for this type of leak.) After checking the windshield and the seals, the problem was actually a cracked seal behind the firewall. When I pulled the carpeting up and underneath the glove box area, I could see beads of water were forming in a small hairline crack where the seal is. Pull the carpeting and the rubber back and take a look there if all else fails. The problem was back behind the firewall, and the water found entry at the point of least resistance, as it pooled at that spot with nowhere to drain due to the design of the car. I had it fixed for about $250.00. No leaking since!

Darline