Hmmm ive got 6 1/5" splits in my front doors, and they fit pretty good..
Wiring diagram? What for... Multi meter finds waht you need..
If you can avoid running the front splits off the head unit and amp them do that, becuase if your gonna have the subs in there, you want the power to your speakers to keep up with the bass. If anything, dont amp the rears, as they are just for fill.
Ive Installed my stereo in and out of my skyline and old cars heaps... so if you have any Questions Just PM me... Good luck