Be careful running the fan with the car off...
That sucker will put a hurting on your battery.
If your doing it for "cool down" at the dragstrip, you really have to be careful.. as it will drop the voltage to a point the car "slows down".
If the battery voltage drops, the alternator will be running at "full charge" when you make your pass {not conducive to running low ets)
not saying it wont help, but those that think if a "little is good, the more is better" will quickly find themselves with a dead/weak battery and the ignition can suffer from the loss of available power.
Do a search on a "tick tok" box, real race cars use them in conjunction with an electric water pump.
First it runs the pump for "your desired amount of time" then the pump turns off and the fan runs for "your desired amount of time" then it cycles back to the pump... and alternates back and forth.. so you dont "wipe out your battery" trying to cool the car down.
Just a tip, hope it helps