the only part that would be a problem is the harness that goes from the body to the liftgate. ( pic 1) other than that i see no reason that you can't take your time and cut the tape of the harness and run new wires then retape the harness. You may be able to actually pull a few extra wire theis way too. then you already have new wires run for future additions. Yes this is timne consuming but any quality job is.
I took apart the intire wiring harness and then completely rewired the PDC on a H.O. (PDC melted down, long story) by just taking my time and double and triple checking the wires before I soldered them. I am also about to wire a couple Street Rods with an I-Pod based stereo that will have no head unit, So when the I-Pod is removed you will no stereo at all. So i know all this can be done if you just take you time.
I think the easiest way, although not the fastest would be to unplug the harness at the tailights and pull it out up to the back doors, then you can easily cut it apart and run the new wires, if you decide to run a connector you can do this now and then reinstall the harness. then pull the new wiers like i described starting from the speaker locations and run them down to the connector. The key to pulling news wires in this way is to pull them as short a distance as possible.
Dingo