By far the most common problem is a vacuum line leaking causing this problem. Check around in the engine compartment for any small hoses that may have come disconnected or cracked and leaking. The hose picks up it's vacuum from the engine, and sends it through the firewall via a small vacuum hose, to the heater control unit.....check real close ,,,and id there is no leak, the control switch will have to come out and be inspected/replaced.
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