If it didn't come from the factory with navigation, that means it's not pre-wired for it. The wiring harness runs from the back all the way to the front, not an easy job, you basically have to remove the interior, on top of that it's very expensive. I think there's a link under the "How To" tab at the top of the page.

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2002 X5 4.6is
1999 C5 Corvette
1992 Mitsubishi 3000GT VR-4
1994 Kawasaki ZX-9R
2005 Mazda 6 Wagon (Wife's)
1991 Mitsubishi 3000GT VR-4