In my experience, the best sounding, most realistic systems, always have a spatial ambiance about them that is created by good frontal imaging and just a touch of rear fill. When listening to these systems, you can close your eyes and almost imagine that you're at a live concert with the stage stretching out before you. The trick to creating this illusion is to locate the sound stage in front of the listener. Master installers accomplish this by positioning the speakers in such a fashion as to provide good frontal imaging. This can sometimes be difficult to achieve in a vehicle.
Space is generally very restricted in the front of a car.
For the most part, dash mounted speakers are limited to only 4 inches in diameter or less. And even if the speakers are Mondo Kamikaze 1000's, it will take more than a miracle to get a couple of 4 inch speakers to produce the acoustic illusion we are searching for. Since most tweeters are pretty small, they can be mounted quite easily in the dash. Installing speakers in the door is another matter. The Right Stuff Installing door mounted speakers takes the right stuff.