You are looking for a vacuum leak. The vent doors are actuated by vacuum. If there isn't enough vacuum, they won't change direction, and the defroster is the default.
Check under the dash for leaks as well as under the hood.
Let me know what you find. NOTE: If you are lucky, with the engine running, you may hear a hissing where the leak is.
Wednesday, June 9th, 2010 AT 7:00 PM