It could be the blower regulator. If you look above the booster you should see 3 wires. I think you want to maybe check the yellow one for power? Do a search on "blower regulator" and you'll find what you need. I had the same symptoms on my dad's 300E. In fact, I have a blower motor I bought from a junkyard for $50. It's yours for $35. You could pay me when I deliver the SD if you wish. The blower cage is broken though. It was fine until is rode around in the trunk for too long and got rather beat up. The fan works though and I think it's out of a '94 400E/E420, (I'd have to check to be sure, but it's newer). It still fits because I placed it in my dad's just out of curiosity.
Thanks
David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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