Is it possible reduce the watt consumption of a 12 VDC radiator fan by using a motor controller?
Buy a DC to DC buck converter from your favourite online market site. There will be plenty of options with an input voltage range that includes 12v. Make sure you get one with at least 7A output current rating, should you wish to use your fan at full voltage / full current. Turn the output voltage down until the fan runs the speed you want.