If the fan runs fine on Defrost, the brushes are probably fine. Defrost puts the motor to default high-speed and bypasses the resistor assembly. If the brushes had problems, the fan would have difficulty reaching full power because it couldn't draw adequate current. Not saying it couldn't use new brushes, but they're unlikely to be the problem.

There's likely an issue with poor soldering on the pushbutton unit, a poor connection in the switch box (module behind the glove box), or a problem in the resistor unit.

If you push the switch to Max fan and it still doesn't run like it does in Defrost, the problem is probably in the pushbutton unit. The Germans didn't seem to know how to make electronics with good solder joints in the 80s. I can't even count how many intermittent frustrating problems I've fixed in my 2 just by resoldering control modules.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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