I picked up a vacuum pump kit from harbor freight for $30, and I've just completed some individual diagnostics on my vacuum pods (pump already paid for itself bleeding my brakes). My 4 outside air pods are good, the center door unit is good (!), but the defroster unit is NOT (I didn't check the heater box). My 1986 (560) supplement to the shop manual (different than the diagram previously shown) shows that this unit is dual; apparently, the lower part contributes to it's operation, and the vacuum for the upper part is shared with the center door. Therefore, if the upper part of the defroster vacuum actuator is leaky, no vacuum will get to the center door. Well, the good news is this one is a lot easier to get to, the bad news is that I will have to drill out a rivet. I have plenty of neoprene diaphragms and the tool, so there is definitely a remote possibility that I can have this working this weekend!

I think someone mentioned that they had their defroster unit replaced by MB with a slightly larger unit; I wonder if the one I have installed was underdesigned.