I know a bunch of other threads are on this topic and all the answers seem to be the same thing. here are some Quotes
Originally Posted by Onehundred80
- "Water can go down the air inlet for the heater if the drains are blocked on the cowl in front of the windscreen. This would be a good reason for it to happen.
Check under the dash on the passenger side to see if there is moisture around the cabin air filter.
If you have another leak into the car from the window etc, then the moisture in the car would show up when the heater went on.
If you have the air recirculating it would do the same thing."
Originally Posted by dtinker
you could have a pin hole leak on the heater core. Does it have any smell, like radiator water would have. The anti-freeze will cause fogging windows like crazy. You will have to clean the windows with Iso Akly to get it off.
Originally Posted by maxcichon
When I wash the car, I always use 35 psi compressed air to blow water out of the little areas (marker lights, door handles etc.) that hold enough to make waxing a pain. While doing this a couple of years ago, I noticed that water wasn't draining out of the wing depression in the boot lid (roadster). There are two small rubber ducts that carry water away and to above the license plate area. Mine were both plugged completely with....wait for it.... pollen.
I noticed a different reason for the car fogging up after sometimes washing the car or out in the rain.
There is a drain hose in the fuel filler area that allows all fluids to exit that area. The hose goes down along the inner fender inside the car and exits out just behind the rear passenger tire. (that is the little black hose sticking out). If this hose is cracked or broken, any and all fluids that get in the filler cap area will enter the inside of the car. It will run along the same path as the hose and exit the car through the same hole where the hose exits so your trunk will never fill up with water. It will soak the piece of foam lining that area.
This is the area I am talking about (the hose is removed in these pics)
