I have a 03 grand prix with the same system - To keep the 4s and the 6x9s you will have to splice a set of RCAs onto the cars speaker leads which run to the amp for the rears. cap off the headunits high level outputs for rears.

Everything else is gravy - also run the kit wire on the passenger side near the floorboard - you will have a straight path with the fuse box wire to the battery.