The easy fix. You can wire the aux fan to come on by the AC system or engine temp. Replace the brass switch on the thermostat housing with a 100°C from MB or other MB supplier. The switch on the drier goes to ground. Wire the 100°C switch to ground the circuit as well.
The two switches work in parallel. When your engine hits 100°C the fan will come on and the temp will fall back to 90°C. The cycle will repeat.
By the way, the switch on the thermostat housing you will replace is there to keep the heater from coming on until the water is warm. Don't want to get a little chill now, do we.

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1984 300SD turbo 126
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