2 questions!

Never had an AC Dart but I believe those vents to be controlled by cable I could be wrong. It's not uncommon for them to come apart or break at connection point for the pivot point for the door inside the heater box to become jammed or broken . One possibility is to separate the heater box by removing the clips around the exterior of the box and look inside to see what's going on . But thinking back my 71 barracuda did have dash pots controlling AC functions either way I still think something's probably jamming or disconnected