If you want to add rear speakers and don't want to butcher the back panels or buy replacement panels then fit 3" 1/4 speakers behind the louvered vent in the back panel. I have just done this and it works well. OK it won't move the car with base but it fills in the mid range when you are driving at speed. It cost me

 

KEEP the stock HU. You do not need to replace it... Just amp the speakers and get a good set of component speakers on top of that. I think my front (and only) speakers were $300 a piece and with them amped from the stock HU on MY04, the sound is amazing. I also shaved the styrofoam emergency kit in the trunk to a much lower height. To do that I heated up a knife with a torch and voila, it cut right through the stuff. I mounted the amp on top of the styrofoam, but in a way so that all of the tools are accessable. I then built a simple box that fit in the rest of the 'hole' now created, and put a 10W0 in it, which is more than enough without trying to go for show and such. Total cost of project: $1300 w/ labor for speakers and wiring (amp was $400). I will take some pictures of everything if you PM me.

 

Thank you! PM sent.