I put a new headunit in the front of my car, a Kenwood KDC-5028...very nice and pretty looking. I just put 4" speakers in the front, but for the back I ripped out the stock 5" speakers and ran the wire to the back and put 2 6x9's in boxes and that rounded out my component speaker system. Shortly thereafter, I put a 16 x 16 x 16 bandpass sub box with a Rockford Fosgate sub on my back seats, I just folded them down, no one sits back there anyways. Then I bolted my amp to the other folded down seat.
Plays real nice...good, clean, crisp sound