buy switch, hook one wire from fan plug to a ground, one to the switch, then the other from the switch to your battery.

cut it on, if the fan blows the wrong way, switch the ground and the power wires coming off of the fan

or instead of putting it on a switch, just run one wire to ground and one wire to something that comes on when the car comes on in the fuse box under the dash.... it will stay on the whole time the ignition is on but you wont ever have to worry about forgetting to turn your fan on.