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.
The link below is a video on the wiring diagram.
Remove 1 2 of covering from the positive and negative wires running into the fan motor.
Run two wires to the fan motor.
Attach one of the new wires to the positive wire and one to the negative cable and solder them together.
The switches turn on the fan at low speed and then high speed as the coolant temperature rises.
Installing a 2 speed fan switch with a radiator hose adapter.
It can be done in any car or truck.
Just an idea on how to get the 2 speed bmw temperature switch installed on a classic gm or american vehicle.