Here's what I've discovered. I have a 1982 300SD. The fan unexpectedly stopped on it. As I repair electronic equipment for a living I thought I'd take a crack at it. I pulled the fan out of the housing below the glovebox. Testing the fan plug revealed that I had 12v coming to the plug with the fan power on max. I ohmed out the fan motor and read 12.81Mohm; essentially an open. I removed the brushes off the motor and discovered severe pitting/scarring of the brush's contact surface. I also discovered that there is an inductor located on the motor board. The inductor actually had a partial break in the solder joint. After resoldering the inductor, despite the pitting/scarring to the contact surface, the fan motor operates quite well now. After the repair, the fan motor ohmed out at 6.2ohms to 13.6ohms.

1982 300SD 262890 miles
1981 500SEL 181922 km