The description says it comes with a "Full Wiring Kit" so I'd assume you have to run the power wire, RCA's and Remote wire from the radio, and ground....
I would just replace the head unit... I did that and it made a world of difference, the stock subs actually now sound like subs, plus you're not sacrificing trunk space.
AVH-X1600DVD - 2-DIN Multimedia DVD Receiver with 6.1" WVGA Touchscreen Display, MIXTRAX, and AppRadio Mode for iPhoneŽ 4S and iPhoneŽ 4 | Pioneer Electronics USA
I have last years model, the 1500 (only difference from this years 1600 is the design of the interface is slightly different)... but crutchfield sells the 1600 for $299:
Pioneer AVH-X1600DVD DVD receiver at Crutchfield.com
Plus, you get the dash kit, wiring harness, and antennae adapter free, the only other thing you would need is the control box for keeping your steering wheel controls (if you have them, or want to keep them...) Here's mine:
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