Related links:
http://freshcapmushrooms.com/learn/keeping-it-clean-how-to-design-and-build-a-laminar-flow-hood/
https://www.familyhandyman.com/automotive/car-heater-repair-tips-fixing-a-blower-motor/view-all/ -- "Most car manufacturers use a series of resistors to make the heater blower motor run at different speeds"
https://www.iforgeiron.com/topic/20309-car-ac-fan-conversion-to-blower/
http://www.resistorguide.com/applications/blower-resistor/
http://www.electro-tech-online.com/threads/12v-dc-30-amp-variable-blower-speed-control.144290/
http://www.doityourself.com/forum/electrical-ac-dc/416524-dc-car-blower-motor-drawing-too-many-amps-what.html
"What do I need to control a car's heater fan blower motor? PWM?"
http://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/
Details from one of the two blower fan units extracted from a car air conditioning/circulation system: