Spend a couple hundred to get an inexpensive portable navigation computer now. Then, start shopping for retrofit parts. The big difference is that the nav controls everything - radio, on-board computer, DSP settings, telephone, etc. So, it is not just the navigation feature, it is a major upgrade to in-car electronics and usability.

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input