12V PWM to Analog DC converter for car cooling fan
If the new fans have PWM control then they can be wired in parallel, PWM too. Can you source this type? It will make your life easier.

If not, the problem becomes that the new fans may not work well depending on how finicky they are about their voltage.

There’s two approaches.


Direct PWM chop
PWM to voltage conversion


The first approach might work if the fan motors can tolerate the PWM chop action. This gives a nice linear control with minimal losses. If they're brushed motors, no problem. If not.. you’d need to ask the vendor or try it experimentally to see if the fans will work with that chop. As near as I can tell, BMW uses about 300Hz chop from the ECU.

The second approach is problematic as the range of control will be limited by the fan stall voltage. And, it may not be practical on a fan that size due to losses in the conversion, unless you use a fairly sophisticated DCDC power supply that switches at a higher rate. It is possible to coax such a supply to work under PWM control.

   
More: did a bit of digging. Siemens / VDO part number 410-65008-01 is a purpose-built module for this. AC Delco makes one too, part number 15-80803.