I had a cavalier that used to let water in from underneath, when cars are built there are holes in the bottom of the body where it sat on the jigs in the factory, these are normally covered with rubber bungs, one may have come loose?
If it is a Ford and it's recently been serviced it is definately an incorrectly fitted pollen filter. They are often misfitted even by Ford techs and water runs in the passenger side under the screen and into the passenger footwell.