Proper surround sound speaker placement is even more important in a car than in your home. This is because there are external sound sources that can interfere with the sound from your audio/video system in your vehicle. Ideally, you would be able to place each speaker the same distance from each individual listener. This way the audio track will reach each person exactly as it is supposed to. This isn’t easy in a vehicle. The easiest way to install speakers is to install them so every occupant has an uninterrupted direct line of sight to each speake,r because everything but the lowest frequencies are directional. Subwoofer placement isn’t as important, since low frequencies are omni-directional.
For more info, see Understanding How Component Car Speakers Work