95 Crown Vic ATC

Out of the blue a week or 2 ago, my heater/ac developed a mind of it's own. It still makes heat or cold but dumps it out everywhere. No matter what
setting, air comes from the floor, dash vents, and defrost. I am betting on a vacuum leak but have no luck finding it, so far.
I plan to follow the plumbing again tomorrow but I thought I'd get a few suggestions here, too.
Anybody know of particular weak spots that might cause this?
One more odd symptom, no changes at all with engine speed or throttle positions. And when I turn it off, the solid thunk of the inlet tent closing is there, just as usual. Oh, for giggles, I ran the ETAC thru it's diagnostics, all A-OK.
Thanks folks!
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