the way i see it, is they dont understand because there are holes for speakers to go.
i used to work at circuit city, and i would tell people you will get better sound out of a single set of coaxials and a cheap 2 channel amp, then if you were to buy 2 sets of components and run them from your headunit. they looked at me like i was a r-tard. mind you, comps were like $200 each set, and a pair of coax's can be had for like 80 and a 2 channel amp can be had for like 120. so for the cost of about one comp set, your shit will be as loud and have less distortion.
they dont realize the biggest upgrade they can make to their output and fidelity is the amplifier, speakers dont mean that much, but the second best thing they can do is build on axis enclosures for said speakers, and the third thing they can do is get components in those locations and aim the tweeters correctly.
imo, the amp makes the biggest difference. but i dont drive from the back seat facing the trunk. so i wont put any speakers there either. and as far as i know, my gti has one of the loudest stereos around - not including bass - and i only use midbass and my horns.
//content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif
rear fill. hah.