There is a way to do it that is not easy, it requires a totally seperate harness that is "jumpered" into the existing wiring. The nav system touches the audo amp system for voice commands, the data link system for speed info used for the GPS display, power from the audio feed and the harness runs from the dash to the trunk!!! What a b%*&h to install!!! But if you are really stubborn, you can do it... I'm sure you can install a good aftermarket unit for a lot less misery. I recommend the new Pioneer systems.
If you decide to not pursue the upgrade, please let me know, I'd be interested in the components.
Thanks, Cesar