Blower Brushes
My heater brushes finally gave up the ghost after several 'bodges'.
Relucantly I took the motor to a specialist motor company who do windings and rebuilding for all sorts of electrical motors, every large town has one somewhere. Refrigeration motor companies especially.
The copper rich compound of the brushes caused a little head scratching, but they found some from a 24v motor that were suitable with a little shaping.
Two hours and £20 later, I have a quiet running heater motor with new brushes and a clean com. Mercedes want approx £160 just for the motor, plus who knows what for removal and replacement. The 'how to' already posted shows what an easy job it is, no more than ten minutes to remove or replace.
My advice if you brushes are playing up, take the thing out and show it to the experts, dont leave it till your 'bodge' causes further damage to the com, i was lucky. Repair will be easier and cheaper than you think, certainly was for me