FWIW, I don't think any 1990 MB has a resistor pack blower controller. I think IC blower controllers were in vogue by 1990. Could be that the W201 is an exception.
The blower motor has two wires. Put a ground to one and a high current pot or switch to the other to manually control the blower motor. Make sure it spins in the right direction.
You can't easily convert an IC blower control PBU to work with a resistor pack blower controller.