Stick with the stock radio unless it doesnt play cds, which would suprise me if it had a tape deck in there. Stock Radios are alot nicer looking and look clean and flush with your car. What you could do it just what bone said and what i did. by an Output converter for like 10 dollars, which splits your speaker wires into rcas, and run your rcas from the coverter to the amp. bypass the stock amp, trust me. The tricky part is your remote wire, If you have the U shaped connector, about the size of a fuse, you could wire a speaker wire to the U shaped connector, and plug it in where the radio fuse is, dont worry you shouldnt blow nothing because the fuse completes the whole circuit in your car. With that wire, run it to your remote slot on your amp.....I know it sounds complicated but its not. THen run your power wire from your fuse relay system or battery to your amp.
If you want to spend 100-300 dollars on a cd deck when you got a perfectly good deck in yoru car, your choice, but id stick to the factory, because it looks alot cleaner and flush with your stock in there.