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.
Yep, I agree. The drains for the cowl inlets may be blocked.