On all bmw models up through the late 90s the auxiliary or main fan is controlled by one or two thermostatic switches that are mounted into the radiator side or bottom tanks.
Bmw radiator fan switch wiring.
Remove 1 2 of covering from the positive and negative wires running into the fan motor.
Run two wires to the fan motor.
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Installing a 2 speed fan switch with a radiator hose adapter.
The link below is a video on the wiring diagram.
Cut the power and ground cables from the fan motor.
Just an idea on how to get the 2 speed bmw temperature switch installed on a classic gm or american vehicle.
It can be done in any car or truck.