I would start with upgrading to a good aftermarket head unit and a good set of components for the front doors (assuming it's 95+), this alone will make a HUGE improvement! I personally would recommend a set of Kicker KS components, best bang for your buck in my opinion. Then find a good set of plate speakers for the rear pillars; Several manufactures make 4X6 components; Alpine, Boston Acoustics, or make your own plates and use a 4" coaxial speaker. Then it is up to you on if you want your speakers amplified or not.
I am much like you, and listen mostly to Rock and Country so I don't need ground shaking bass. However, adding a decent sub and amp combo will REALLY tie your entire system together. I would recommend a sealed enclosure, built towards the smaller end of the recommended box volume to have more of the punch bass that rock music typically sound best with. Again, I will recommend Kicker for the Sub and Amp(s). Kickers amplifiers are great, DX or ZX series will leave a smile on your face when you turn it up!
Also, UPGRADE YOUR SPEAKER WIRES!! It will make a BIG difference in sound quality, over the stock speaker wire. It's a little more work, but is very very important in any real system. When installing an amplifier, use good quality wire through out. So use braided RCA's, Good quality power wire, have good grounds throughout, I would also recommend doing the "Big-3" ugrade regardless. Also be sure to run the RCA(s) and power wire(s) as far apart as possible, to help ensure no noise gets in the amp signal. And most importantly, once you get everything done; Sit down, and turn it up!!!! :Cheers:
