@OP: i'm stuck with the autotemp II on my '79 SD as well. i found a working servo and amp at the wrecking yard. so far (3 months) it's been working like it's supposed to. i actually went ahead and coated the plastic portion of the servo in epoxy in hopes to prevent any cracking; only time will tell if it holds up. i'm still planning on doing a manual climate control swap once i source all the parts.

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1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL