I've done this (rather crappily... so crappily I removed it) myself, so I know a decent route to take. Run the cord from your headunit down over to by the foot rest. Remove the trim over the weather-stripping on the floor. It will just pull out. Then, using a nut driver, unscrew the plastic nut holding the corner trim to the car, and remove it. You'll see LOTS of wires. Run the lead with them. Get into the back seat, and again pull up the kick plate (if that's what it's called

). Wiggle the wire through the B-pillar trim, through to the black trim by the seat. Tilt the rear seat forward slightly, pull out the rear tray, and finagle the lead through to the spare tire. Then, using a 10mm nut driver, undo the top tie-down, and pull the trim away from the body, doing the same with the trim around the window. Just pull it slightly away. You can bring it all the way to the headliner, and to the rubber weather-seal. I've never gotten a wire through where the OEM gets the wires though (that rubber tube thing), but all I can say is goo luck. Pull the trim off the hatch (it will just pop off). Unscrew the ballstud from the driver-side WINDOW strut. This will give you more room to get a wire through the little channel. I used a bit of metal wire off a spool (not the electrical type of wire) to get something UP the channel to attach to the electrical lead, and carefully got it into the main area of the hatch. And there you go!
It looks confusing and intimidating, and no I don't have pictures, but good luck!