There is a simple way to address the problem- - - -use a speed control resistor designed for multi-speed fans on 12V systems that mount inside the air box with air flowing over it to control the heat generated. By connecting the various speed control windings in parallel, you can get the speed you're looking for. Virtually all 12V climate control systems have those reisitors- - - -go junkyard scavenging and find one you can adapt to your application.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!