When the defroster is on, the AC comes on to help dehumidify. That may cool it some but not make it cold. I believe there is a blend door on your vehicle that may be causing the problem, but you need to rip into the HVAC duct work to see if it's working correctly. It will be located near the heater core.
Let me know what you find.
Joe
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Tuesday, January 20th, 2009 AT 8:34 PM