Now back to reality with some details. The parking light switch provides power to the parking lights either from the parking light relay controlled by the ignition switch and light switch stem or directly from the battery when the switch is on. To repurpose that switch, you'll have to permanently connect the parking lights to the parking light relay and run a wire from the switch back to your fan.

I think you were looking for 12v power already in the back near the engine. Depending on the thoroughness of your harness dieting, you may be able to use the trunk light, rear window defroster, or rear accessory power. My guess is that all those are long gone.

I still like the parking light switch solution. It lets you run the fan for a while to cool the engine down after you shut it off.

RPG