I'm not sure if you can fit it in the CD spot - I know someone has done it ebfore but It's a pretty tough mod. If you want to hook it up directly to you car speakers (no headunit/radio), You'll need an amp - which will not fit w/ a carputer in your radio slot.
One of the most common installs I heard about and is pretty easy is underneath the seat.
Are you considering installing a screen? If so, what size? The screen might fit where your old radio was - if it was a double DIN, or the "big radio" (one where two standard CD players could fit in the slot, if you know what I mean).