The tricky part could be the GPS antenna itself. If I remember correctly, the wiring diagrams showed that to be in the same antenna cluster on the roof of the vehicle with the sattelite and AM/FM antennas. The stock antenna run goes up the A-pillar on the driver's side and under the headliner to the rear of the vehicle. If that's the case, I'd be hesitant to just tuck wires up there without being concerned that they could interfere with the side airbags, and I don't know how tolerant the headliner would be to peeling it back to find suitable anchor points for the antenna. I'd imagine that the bulk of the installation time for the dealer would be in performing that portion correctly.
An easier option might be to mount the GPS antenna on the dash, at the risk of not having as good of a signal.
If you do install it yourself, please take some pictures of the back of the radio as well as any other hidden things you come across. Thanks!