Very tiny black plastic vacuum line running across the top of the firewall. That feeds the climate control assembly. Check that line for vertical breaks or horizontal splits. That's the most common place for a vacuum leak. There are other places of course.
Here is a diagram of the HVAC vacuum controls, courtesy of TMoss (Tom Moss) and veryuseful.com:
http://www.veryuseful.com/mustang/te...m_controls.gif