A couple years ago I installed some 6 1/2" stock subaru speakers behind my B's seats. I used the pattern the speakers came with to cut a hole in the separation from the trunk to the cabin, and slipped the speakers in with no problems. I'm sure you could rig something with a bad-ass woofer if you wanted, too. This setup doesn't take up much space from the trunk OR the cabin.
I also added some 1.5" peeps on the dash and some alpine speakers in the doors.
In sum, 6 speakers with a full range of tone. No amp was needed. As for quality, my speakers are all fairly cheap, so they definitely leave room for improvement, but I really like the "surround sound" effect.
As for wiring, I am not an electrician (despite the years of tinkering with our good friend Lucas) and simply made each side in-line. No fade, in other words.
I believe this is a good setup if you want to keep things cheap, and don't want to clutter up what little room you have.