I had the same intermitent problem on my 92 300CE. I did replace the 30 amp strip fuse which was cracked in half and all was well for a few weeks. Then the fan quit working all together regardless of the setting. The fuse was fine. It turned out the blower motor was bad. Had it replaced and worked great ever since (one year ago).

That motor is not cheap and you need to be very careful removing the squirrel cage fans if you want to reuse them to save $$

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1994 E320 Cabriolet
1992 300CE (Sold)
1991 300SE (Sold)
1986 560SEL (Sold)