I think you also need to know the frequency at which the fan operates. Maybe that can be done with a square wave generator (just a guess).

Here's a document with some good info on an aftermarket PWM controller that can control both OEM (GM) and Spal fans. There's also a diagram on page 13 that shows how to wire a single or dual fan setup.

Link: https://www.lingenfelter.com/product/L460320002.html Go to bottom of page and click on "View Instructions" button.

Hope this helps