I had this problem last year. After a hard rain I would have an inch of standing water in my floorboard.
The water would drip from behind my glovebox.
Take a water hose and run water down the windshield and into the plastic grill at the bottom of the windshield. If you start to get water dripping behind the glovebox, then the water is coming in at the cabin air filter.
Click here on how to get to that filter.
Click Here When you get under the grill, you will probably see how the seal around the filter has deteriorated.
