a/c vent door problems 08 mustang

Sounds like the door may be vaccuume controlled. When your vac supply is lost the vents default to defrost. Check under hood for vac lines that may have feel off. There is also a vac accumulator that stores/builds air pressure. Usually those are in the fender well

i have the same problem . the ford dealer said there is no vacuum on the ac system all the doors open and close electric motors. they said it will be 90 dollars to tell me what it is.