Hey this is very urgent, I need to get some info on this.

I'm getting my double DIN unit tommorow to install and i've installed all sorts of a/f headunits but the sc3/400 seems so complicated wiring. I don't know what i'm supposed to hook up because of the factory amp and sub and ****. I wanna use an aftermarket amp and subs AND the factory sub and amp is this possible. Also I wanna use the factory speakers just change the headunit and hook up subs and amp thats all I wanna do. But all these wire diagrams are confusing I get the power/acc/ground and antenna but the speaker wiring is where it get's confusing so if someone could please clear this up with me with color codes of some sort or something and what to do to keeep the factory amp it would be most appreciated thanks. Oh yeah I have the system with the 12 disc changer but not the premium it says nakamichi on the changer so. (1996 Lexus sc300 5 spd)