The cheapest path is to buy TCU part number 84.10-9 195 455 from an online dealer like Silver Springs BMW. They sel it for $338. There is a core charge, which means you need to send your old one back.

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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