Sounds like you have vacuum-operated controls and the vacuum line is cracked, broken, or disconnected. When that happens, for safety, the systems are spring-loaded to go to defrost mode. You might have cold feet, but the windshield will be clear. The clue is you will not hear any sounds from the dash when you press the buttons to select the other modes.
If you DO hear things moving in the dash, suspect a broken actuator arm or linkage. Any hissing sounds heard only when pressing the buttons suggests a leaking vacuum hose going to one of the actuators.
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Saturday, April 30th, 2011 AT 11:30 PM