not a mopar question, 09 chev uplander

Take the heater core feeds off and blow compressed air through them. I did this to my 01 silverado work truck and it blasted the black engine cover with a vanilla milkshake substance that is still on it. It heated better after than clearing. Sounds like maybe a vacuum blend door issue but I would think electrical on that later year.. Does your vents operate after you turn your car off? They should hold vacuum for about 10 seconds.