It was the hose in the dash. I pulled the glove box and that hose was all but disintegrated...there are two rubber hoses that end about 5" apart and they're connected by what looks like foam pipe insulation, and that pipe insulation was crumbled to bits. So I bought a length of it for $0.97. Haven't put it in yet because I have no duct tape in the house (flame suit on) but the heater worked properly when the inside of the dash was cool. But when it got hot in there, the heater would blow cool air.
Monovalve good, acc unit good (thankfully), aux pump good, foam tube gone.

Thanks for the help, this site is invaluable.
-Geoff

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now