My 300SD just did the same thing. No warning no unusual sound, it just quit.
Checked the fuses, power at the motor plug is good and the a/c clutch engages etc.
The blower motor was toast. I had hoped to get by with replacing brushes but the armature was completely worn out.
A new Bosch motor should be here tomorrow.

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Darrell

1984 300SD "Ole Girl"
207K