Looking over the possibility of replacing the OEM navigation with a Garmin.
1) Remove dashboard, radio, navigation display, navigation DVD drive.
2) Disassemble navigation display frame.
3) Modify frame to take a Garmin Nuvi.
4) Unseal the Garmin's casing, find the power and antenna.
5) Wire the Garmin antenna into the factory antenna (located near the windshield inside the dashboard).
6) Wire the Garmin's power lead into an ignition-hot line.
7) Reassemble radio, navigation, dashboard.
Pros:
Garmin interface is about 1000x easier to work with. No exaggeration.
Opens up the secondary glovebox for storage.
A new Garmin might even have voice commands.
Cons:
It won't quite look factory, and certainly is not. Pop up navigation system mitigates some of the appearance.
Garmin display is lower resolution, and map display isn't as good.
Other considerations: it might cost less than $20 to repurpose and modify an old Garmin; but a new larger Garmin (with voice cmds) would push the cost into the $2-300 range. Also, I've barely had to use the navigation, because, although I've been doing a lot of roaming, even my roaming rarely takes me away from places I know. It is rarely used. However, when I need it, it takes 5-10 minutes to input an address, and it cannot be done while moving. A Garmin takes about a minute to input an address, and it can, in a pinch, be done while moving.
I do have navigation on my Thunderbolt, but it also trails behind the Garmin interface.