I don't know if you will be taking your motor apart or not since it is out but my 84 300D has a Siemens motor in it instead of the Bosch Motors that others have.
On the Siemens Motor the brushes are up front and you need to remove the blower cage/fan to get at them. Unfortunately I cannot remember the details on taking it apart as I did it a long time ago.
If it has a Bosch Motor there is a few threads on taking them apart. The Bosch Motors have the Brushes in the rear.
This does not cover the whole tear down. I do remember that in order to change the ball bearing I had to drill out some rivets and there was also dirt inside that was binding it a bit.
http://www.peachparts.com/shopforum/diesel-discussion/308273-need-advice-removing-84-123-blower-motor-fan.html