I bought my 1993 300D 2.5 last spring and the heater was great. The AC has been great all summer. But with this first cool snap yesterday, I noticed I had no heat. None, nada. Fan blows, lower vents work, just no heat. On the way to work this morning, I unplugged the heater (mono) valve and still get nothing (even on defrost).

I'm stumped. Where should I look for a problem?

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer