Out of the 4 cars I owned as a teenager, I had to replace the factory radio that was in them. Either because the original one failed or was just a plain joke. I was inexperienced with installing a radio in my car but was determined I couldn't afford to pay someone else to do it.
That was about the time I discovered Crutchfield. Their catalog was filled with possibilities for what car stereo I could afford and the options I wanted in a car stereo. And when they sweetened the pot by having a cross reference for what ones would fit in my car, installation kits AND full instructions (including an 800 number to call for help), I was sold!
These days, their catalog includes a LOT more than just car stereos. They offer great deals and free shipping on most orders too.
Check them out at http://www.crutchfield.com/ .