Thanks for the info. That car has a host of other issues. I'll be sending it off to the mechanic on Friday or so, but I won't have him look into the climate control issue. I'll diagnose it using your tips and then if I fail, I'll send it off to him.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver