check the vacuum line to upper intake (from the tree that leads inside the car and to the vacuum reservoir... the little black ball in the passenger fender well)
The following are diagrams courtesy of Tmoss & Stang&2birds
Vacuum diagram 89-93 Mustangs
HVAC vacuum diagram
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring; http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.