Did you read the post above yours from yesterday? same problem it appears. Turns out to be a corroded/ cracked fuse The post title is "300D heat always on"

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.