Need more info. The Bosch Motor has the brushes in the rear and there is I think 2 threads on how to get into them and change the Brushes.

DIY Links by Parts Category - PeachParts Mercedes ShopForum

If you have a seimens motor the Brushes are up front and you need to remove the Blower Cage to get at them.

Also when I took mine apart it had a lot of dirt and dead bugs inside rubbing against the motor innards.

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84 300D, 82 Volvo 244Gl Diesel