Well, the reason I asked about a tap at the termo is b/c Benz uses a 100C to ground sensor for some models...you could just screw that in and run the ground wire from a relay coil side to that sensor..then use relay contactor side to trigger a full 12v to the Fan..that will give you High Fan only..
I would even use the relay that is already there , behind the fuse box.
That way you alreardy have the relay and fused circuit..just need a sensor.

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A Dalton