Okay, I found what I need to do.
I can get one of those schoche in-line convertors, like pictured below:
and with that, I can wire into the rear speakers at the head unit, into the 'input' on one side (Splice two additional wires into the original harness, and plug those wires into that box), than run the rcas to the 'output' on that box. It may be easier though, instead of putting it behind the head unit, if I move it closer to the amp and subwoofers and connect directly into the rear speakers, than I won't need to run so much extra wire as the rear speakers are in the back anyways.
From there I hook the amp up regular and use the chassis in the back as ground. One question I have though is, since the battery is on the passenger side, I am going to run the amp 12v wire along the passenger side, but is there a good hole in the firewall to run it out of, or do I have to drill a new one?
Some idiot before me installed some cheap fog lights inside the grill, and I removed them, but the switch is still there, so I am going to hook the remote turn on wire to the switch so i can turn the amp off and still listen to the radio when I don't want the amp on (My wife likes to listen to rock and stuff too, so that'll help)
What do you guys think?