Sounds like you have a vacuum leak ( with no or low vacuum controls default to defrost only ) The line from the eng. should have vacuum on it ( 16 to 18 in. at idle ) you can cut this line and test with a gauge ( use w/washer hose to splice it back ) . The vacuum is pulled from the intake manifold at the T/body dr. side and will goto a res. tank and and then inside the car.
HTH
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