I think I finally have this resolved. Unplugging the aux water pump and cleaning all the fuses made no difference.

I bought and installed a new in-car temp sensor then removed, cleaned and reinstalled the little blower that pulls air across the sensor.

So far it hasn't pulled it's 5 seconds of warm air trick. My fingers are crossed that it won't start up again in the spring.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire