If you're getting significant condensation but NOT smelling the usually very obvious smell of engine coolant, seems to me that there would most likely be a problem with the recirculation flap (so that you are just recirculating cabin air instead of bringing in outside air). If your heater core was leaking to the point of causing water vapor to condense inside the car, you would HAVE to smell coolant.

I have read that monovalves DO fail in the closed condition. Monovalve failure is so common that you can get a repair kit (to replace the rubber diaphragm inside the monovalve) instead of having to replace the whole thing. I am far from a climate control expert, but from what I've read over the years, it's a cheap, simple fix so might be worth starting there.

The other possibility, which you voiced in your first post, is that a vacuum line got disconnected under the hood. I know that when my evaporator was replaced, that happened. However, the system is designed to default into full heat/full defrost mode so this may be less likely.

The CD-ROMs have a nice, if tedious, procedure for testing the climate control system and locating faults.

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Steve
'93 400E