In my opinion, i'd screw the rear speakers. Just buy a pair for the front. I'd then save up just a bit more and buy a sub and a 4 channel amp. You'd just need to spend a bit more. Also, i'm making my kickpanels this week. If i got a chance, i'll take pictures on how to remove the door panels and all. I know someone already has done it, but just incase kinda thing.
Ren