Right, the best way you can ever do this..........

Firstly, remove the fan plug, switch the ignition on, and test for which wire has 12v. Once you have located it, on the OTHER wire, you need to run ONE wire back to inside the car, connect it to one side of your switch, and run a wire from the other side of the switch to any earth point.

JOB DONE! The fan will still operate as normal and turn off with ignition, and you can over-ride at any time.

The principal behind it is the fan swtich switches the earth, and all you are doing is putting a switch in the car that connects to earth.

The wire needs to be rated at about 30 amps to match the fuse rating