i tried it before with a 5V fan to cool the chassis when i using TB01 tub which is fully enclosed up. it do cool a bit. the source comes from the motor whcih is connected to the EXC. when u run, the fan spins and it spins faster as u pushed the trigger more. if u connect it direct to the battery, it will always spin even if u are not running, waste of battery runtime.
Or u can buy those Pc fan, 12V and use the batteries after run to power them to cool the motor and esc after a race. after 1 or 2 heats, both of them will be cooled off. When u buy the fan, look at the blazes and the Amp rating. they will affect the amount of air that is blown into the stuff that u want to cool
Just my advice.