Rodneym:
A good way to do it is to hard wire the fan through a High quality 40 Amp 4 prong normally open relay and control the closure of the relay with a thermo fan switch in the cooling system that is on a fused circuit and carries a manual overide. I have a Fluidyne radiator with the fan switch in the lower section of the RH tank, with the key off if the fan circuit is closed (over 193 degrees) the fan will run for about 3-4 minutes as it cools it down, at which time the fan switch opens and the fan quits running (unless in manual overide).
PS...The Rug did NOT go in the car!!!