Unless you are exposing the player to unusual amounts of heat (i.e., leaving it in the parked car on a hot summer day), you should not need a fan.

If you install a fan that is too large, it will get in the way of the shock-mount protection of the disk drives, which is potentially a worse problem than the player getting a little warm. You're better off just taking the player out of the car and bringing it with you when you leave the car.

Specs for the temperatures are listed here.