Steve-
Thanks for your (quick) reply:

<<and probably the most important it has an over ride switch which controls the motor so that the servo can be manually driven to a different position.

Really! does that mean that the servo could be "driven" to the no heat position and disconnected? I'd rather have cool than heat.

<< From your description you are either stuck in full heat or defrost. I would imagine that you have a bad amplifier (behind glove box). I would have condemned the servo first but you did say it was replaced??

full heat sounds about right. This puppy puts out great heat (last thing I need today!)
Servo replaced in 95 or 96. I remember it's being a fairly pricey chunk of hardware. ~$400
I don't recall the amplifier's being changed, but will try to find receipts.

<<Find that fuse on your wiring diagram (I don't remember where it is in the 123 car).

I'll check again. I have the diagram at home, but it comes off of the main fusebox (in the diagram) as a small guage wire, and heads straight off to the amplifier. I'd sure rather replace an inline fuse than an amp and/or servo. Probably the logical thing to do next is to check for voltage at the amp.
I assume I should see 12v at the connector attached to the colorcoded wire shown in the diagram (assuming it's accurate). I see no indication of any switching between the fuse block and this mystery fuse. Time to pull the glovebox.

ac

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97K '78 300D