Thanks for all the info. The connection by the broken off part in the picture is so corroded that the fan stays on all the time. I ordered a replacement from Fastlane, around $20. But the screws appear to be corroded onto the switch, probably going to have to drill them out.

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car