In this case, you are looking for a vacuum leak. The air flow is determined by vent doors in the HVAC system. If the vacuum pressure isn there to move them, it goes to a default, the defroster.
Check under the dash for leaks. If you are lucky, you may hear a hissing sound where the leak is with the engine running.
Joe
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Friday, August 13th, 2010 AT 9:47 PM