pulse width modulation car fan
In principle, it's a buck converter of course. The alternative is in using the motor inductance as energy storage (connecting a fast free-wheeling diode and a PWM switch only) or use an external inductor and possibly a filter capacitor, supplying DC to the motor. I guess, the first variant is used in the manufacturer design, but it should be clarified, that the motor can withstand the involved additional AC current without overheating. Also this "direct chopping" method involves considerable interferences. At best, the motor voltage (waveform and frequency) should be checked in the original design.
Or just try. A PWM frequency of a few to a few ten kHz should work.