You can retrofit the MKIV navigation into your car. I'm not an expert, but I suppose you'll need:
- replace the popup box with the nav screen part.
- replace the radio with the nav-enabled-controller one.
- get the mkiv module, which goes in the trunk's left side.
- get all the cables to connect the nav with the radio and the screen on the front.
- get the cables for the antennas (gps)

The system is all the same for all X3. The screen is similar to the 1-series (altough their nav is iDrive). The MVIV nav is the same on the recent X5 (e83), the 3-series E46 and the Z4.

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