before you spend the money, are you positive that a 04 head unit will be able to hook into any model under 2004 (like your '02)?
I'm not really sure... I could have sworn that I once read somewhere that more "upgrades" were needed (ie more money you will need to spend).
I think it was something about different kind of plugs on back of the 2004 headunits requiring you to run more wire, and some other "equiptment" you had to buy.
I could be wrong.
...but if I am right, then you might be better off spending the money on an aftermarket headunit (which would be better sound quality anyway).