The mode control for the the climate control panel doesn't respond. It's stuck in defrost. From what I undestand, that's the position it goes to when there's no/insufficient vacuum. I'm trying to trace the supply back to the source, but can't figure out how it gets from the control module back to the engine. Can someone help me out??

 

There's a vacuum reservoir behind the battery box on the passenger's side of the engine. Start from there and follow back.

 

Thanks Quintin!!!