After looking at this again, I learned a bit more, but I still have no heat. The servo does indeed return to the "park" position electrically after the ignition is shut off. The power comes from an inline fuse (in the fuse box but not in the rows). The fuse in the holder is a weird one, doesn't look like any other fuse I've seen before- a tiny glass tube with metal ends and green rings around it.

Anyways, As soon as I re-seated this fuse, I heard the servo whirring itself to the park position. So that's good. I also notice the aux water pump is moving coolant now with the engine running. Also a step in the right direction.

I tested the ACC servo unit as well. The valve opens when I apply 12 volts straight to pins 3 and 5 (simulates fully open DEFrost setting), then it closes again when I reverse the polarity. The measured resistance at pins 1 and 2 is exactly as it should be for any given setting. So the servo seems fine. But switching on the ignition and selecting DEF at the console, I would expect the connector side to deliver 12 volts as well across pins 3 and 5.

I only get 8 volts.

Does this mean the amplifier is shot? Or does this suggest a bad connection at the selector switch?

I feel like I'm very close on this one. Anybody have a clue?

jk