Definitely try this in the Car Audio forum on this site (use the drop-down box at the bottom of the page). Your car probably has dash-mounted speakers at the base of the windscreen. Check if these are working and delivering the full range of frequencies. If so, there's no door speakers.
Sorry I can't help with the rear speakers except to say, don't worry so much about investing money in them, invest more in front speakers (even if only dash ones). You don't get a lot of bass in the dash, but a wonderful soundstage that really sings using good quality speakers. Recommend Boston Acoustics for good mid-range and treble. I hear MB Quart can have a little recessed midrange, so you'd certainly miss out using those.
You don't usually hear much of what's coming from the rear, so just get something that should make OK bass, as that's all thats worth having.
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black