If you're interested in one them fan gizmos @wheelsquad
lemme know. It would be simple and very inexpensive to build. There might be anywhere from 1-4 parts required... a zener diode, resistor, transistor and a relay should do it. But I'd probably get it down to just two or three parts after some testing... and after sleeping on it. It would probably cost more for weatherproof wire connectors and a tiny enclosure than it would cost for components. So maybe about $10 or so...?
Carrying it one step further and building a complete fan control unit (adjustable factory replacement unit) that included failsafe features could be done for a few bucks more using the same 2-4 parts to control the low side. Hollar when the snow is getting deep... I'll be looking for stuff to do.