Most likely you will be in the market for a new MKIV nav computer. There is not a separate, external GPS receiver like with the early MKI and MKII nav computers. Since the GPS receiver is an internal component, you could probably harvest one from a dead MKIV someone is selling. Otherwise, you will probably just need to get a good MKIV and start over.

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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