Speaker placement, 91' Accord

For proper sound stage imaging, the sound should appear to originate from in front of the driver not behind them, so your best speakers should be mounted in the front of the car.  When you listen to a concert, or watch a play or any other live event, you usually face the source of the sound.  The best car stereo setup attempts to simulate this.  Now, that would mean the best location for your speakers is in the windshield, which is not really practical.  So keeping the source of the sound as forward as possible is the next best thing.  Center channels can also help.

That's the theory, in a nutshell.  There is actually a lot more to it than that, but hopefully you get the idea.  Bottom line is to try and shift the best sound forward in the car's cabin.  The rear speakers really only need to be for fill and ambience (or surround effects.)  The sub is largely non-directional, so if you use one, it can hide anywhere.