Had the same issue on my 300E. Blower would squeak once in a while, thought I'd get around to lubing it. Then it squeaked again and at the same time slowed down and dimmed the lights. It's way of saying it wants attention, NOW. Bearings were stiff, brushes were worn down, but the clincher was that the commutator was also noticeably worn. $60 and a few hours later and all was as it should be.

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon