The best place for a fan, is on the other side of a duct from the motor. The interior of a car is confused and turbulant. "where" the fan is, has no real bearing in there. Being confused and busy air, any stream of air from a fan, is going to be disapated quickly. so you want it close to the motor. A duct keeps the airflow attached to the motor as much as you can, giving you the best cooling you can get.