When your ac/heat control system loses it's vacuum it switches to the default position which is defrost. As said above--look for a vacuum leak. Possibly the pump, vacuum reservoir or loose hose.
DICK
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2010 Phaeton QTH 40'
2017 GMC Sierra Crew Cab pickup 4X4
Recon, D,2/7, 1st Cav '65
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