well most of the time we listen to it while we are not moving the car, in real case when you are travelling, the road noise and external traffic noise will obstruct the front good sound travel to rear passenger.

I used to only suggest front, but upon real world testing, the rear just wont have the enuff amplitude of music reaching their ears, so a rear speaker is actually good idea if it's for rear passenger pleasure. Especially for kid when they need to listen to the dialogue in their movie.

Just make sure the rear spk were adjusted with proper gain which do not affect the front..and you get no complaint.