That seems like a terrible way to do it. So it's just a variable controller. Except electric fans aren't generally rated for variable speeds, they're rated at one speed. And the fan itself will be designed to be most efficient at that rated speed. That also means the fans are always on at some speed once you're within 10° of the thermostat open temperature? Again, there's no reason for that. The fans don't need to run at all once the car is moving at a reasonable speed, and the temperature won't necessarily drop low enough to shut them off even though you don't need them to be running. Not only that, but most electric motors tend to like a specific RPM range. Not all fans may like being hooked to a variable controller like that. Big pass on that one.