Originally Posted by
onehundred80
The most common reason this happens is when the water drain hole in the cowl in front of the windshield gets plugged and the water rises over the level of the lip for the air intake for the cabin. Clean the drain and drain hose and it is solved. All you have to do then is dry out the car.
You have to take of the windshield wipers and the black plastic cover and all will be revealed.
Spot on. Not the only cause but the most common. I have my fingers crossed this is your problem as it is an easy fix.