yes you can use your stock stereo to hook up an amp and sub....you are going to need a line output converter though...you can pick one up for like 26 bucks at best buy.
as for the door speakers you will have to remove the door panels and then there will be 4 bolts holding the speakers in and a harness that the speaker plugs into....you can also purchase these harnesses for the easiest install from best buy for 12 bucks a pair
^^^he said it all. Go pick up a Line output converter(LOC) and splice into your rear speakers. Then your RCA's are done for the amp. Everything else is normal.
Speakers are pretty straight forward as well. Take of panel. There are about 6(i think) torx screws around the panel, then one by the handle and then two bolts behind the lower handle. This should clear it up for you. [thumb]