More than likely the brushes are just worn down in the blower motor. Do as Sugar Bear suggested and test a light bulb in place of the blower motor. Use a high wattage bulb like a headlight to put some load on the blower transistor pack. If the bulb isn't dropping out, you can be rest assured the motor is your culprit.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]