How do you power an electric fan on a classic car?

For a temperature activated switch, Hot Rod shops like Speedway Motors in Lincoln, NE sell screw in models that go into the engine block or intake manifold by thermostat. They turn on and off current from a fused wire from the battery, through the switch and to the fan. By reversing the ground and hot wires on the fan, it will reverse the rotation enabling the fan to push or pull air.