The best resource I ever found, and then bought stuff from there cause they were reasonably priced for the do-it yourself car stereo system installer was
crutchfield.com
They have been in the audio business since the 70's I believe, and really know their stuff.
I obviously don't work for them so that's not the reason I'm touting them, but it's because I have had very good luck with them in the past, and they have always been a great resaource to answer questions for the Audiophile in all of us.
I need to upgrade the stereo system in my new car (04 Cavalier) and will be heading to them and their catalog and webpage sometime over the summer to do so.
Speaking of catalog, they have a place where you can request one for free.
Check it out.