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I can speak at least to my 85 300SD, which should be sufficiently similar.
There are two switches to activate the aux fan both are activated by ground contact.
1. The first in on top of the coolant thermostat valve. A single wire is grounded upon the heat tempurature.
2. The second is on top of the A/C drier. There are two wires which, if crossed when the ignition is in the on position and the A/C is on, will complete a ground circuit. If you jump these two wires while the A/C is on then you are possibly looking at a bad aux. fan. Try jumping the wires quite a few times with the fan completely stopped between to ensure the brushes don't have bad contact points.
Hope this helps.
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