You want it to run even when the car is off? Or you just want it to run when the vehicle is running? If the latter, wire up the fan to an ignition source, so the fan will run whenever the key is in the run position. If you need the fan to run a certain amount of time after the vehicle has been shut off, the Tavarish YouTube channel talks about some sort of relay (can't remember the name) that he will use to run an oil pump for ~30 seconds after engine shut off. I think it's in the video where he mounts the bumper on his Lamborghini.

 

A toggle switch or ignition switch will not last due to the amperage, you must run the fan through a relay.

 

Use a time delay relay to run after ignition is off. Theres diiferent length time delays you can buy...