"Check the vacuum at the two vacuum hoses going into the firewall on the passenger's side. They both have check valves on them."

This one is the most important test and will take about 2 minutes and you can do without any tools.

"With the engine on, they should both show the same vacuum level on a gauge. With the engine off, if you pull the check valves off the lines going into the firewall, one will make no noise (this is the one going to the dome light sensor) and the other should hiss since you are dumping the vacuum. If this is the case, then your leak is on the dash side of the firewall. The one line goes to the 4 ball vacuum reservoir on the passengers' side, from there, the vacuum goes to the electronically controlled solenoid bank next to the reservoir that then distributes the vacuum to the specific vacuum bellows in the ducting via different colored vacuum hoses based on what your dash knobs are set to."

Once you have figure out if the vac issue is inside or outside the cabin it will be much easier to diagnose.

Onkel Udo