If you are not able to open or close the center vent in the dash of your AC this very well could be the problem. Over time this lever deteriorates and the little vacuum actuator that they are connected to, is too strong and they often break off. This lever fits 69-70 Ford Mustangs and Mercury Cougars. 

This part is a hard to find and an exclusive part with Mustangs, Etc. featured in Mustangs Monthly, see article here.