1) new headunit (Tape, Cd, Mp3, DVD, however much you wish to spend) and adaptor plate to fit it.
2) Replacement door speakers (about 5 inch i think)
3) replacement Rear speakers (6x9 by all accounts, plus a bit of mdf to mount them propely)
4)An amp to run the speakers although the headunit all can do this
5) lots of speaker wire
6) and finally a lot of time and patience to take your lex apart to run all the cables, change the speakers, etc, or an audio company that knows how to do it for you.
I did it all myself and it's not too hard. Search around the website and there's plenty of guides to taking door panels, rear seats, etc out.