Have you pulled the blower out and actually tested/checked the motor? They do wear out....in the C-class they are on pretty much all the time at at least a medium-low speed usually....so 12 years of that will indeed wear it out.

Its behind the glove box/passenger foot well cover in the W203....you can see a portion of it under the hood if you take the air filter housing above the battery out, could turn it by hand and see if it moves freely.

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