If you have rear wiper/defog, there should be a flexible conduit running from the liftgate to the upper hatch frame - run your speaker wire through there.
Run 12 or 14 gage - if you don't, you'll probalby be doing this again when you upgrade speakers and add an amp later (if you're willing to do this much work, I'm taking it as a possiblilty. Besides, I just hate doing a job twice!)
You should be able to pull the wiring through the flex tube, then get it down the passenger doorsill just by pulling trim panels and leaving the headliner in place. Why the passenger side? No power lines - just a clean run. The turn signals, brake lights, and other things that are likely to flash run down the driver's side, and you may pick up some "click" from your lighting there (I've noted it with cheaper setups.)
From there, you can figure out the best way to get the wiring to the back of the head unit. I shall also leave speaker mounting as an exercise for the apt pupil - but if you're going to do this to expand your low-end sound, I'd suggest using rubber "well nuts" to mount the speaker - the offer the additional advantage of cutting down on the speaker housing vibrating against the liftgate shell. Do the same thing up front - I've found high frequency rattle to be quite irritating...
I've also found small bits of heat-shrink tubing useful for projects like this - colour-coding the ends of the wire pairs makes it easier to keep track of what went where...
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