Why does the floorboard get wet on my 2004 Pontiac Grand Prix?

Answer 1

Check the cabin air filter, if it is wet after it rains then that is where the water comes from. Directly below your cabin air filter is your heater blower motor which will also be wet along with the blower resistor next to it - not good! Remove the plastic trim under the glove box (3 plastic push clips to remove) and see if the insulation on it is wet. If so the water is coming through the cabin air filter.

The plastic trim that covers the cabin air filter distorts and doesn't seal across the base of the windshield. Look at the black plastic piece that covers the cabin air filter under the right side wiper blade (hood open) It has a rubber strip that seals along the base of the windshield. If the plastic is distorted you will notice a gap that lets excessive water enter under the plastic trim. It then drips down on the cabin air filter then straight through the blower motor past the trim under the glove box and to the floor. A dirty cabin air filter will cause more water to enter than a clean one when the blower motor is running but I wont get into that explanation here. Replace the cabin air filter and seal the gap between the rubber lip and the base of the windshield on the right side of the car.