Read this... it will explain your problem. You have a 99% liklihood that your MKIII navigation computer fried when the battery was disconnected.

Troubleshooting blank nav screen and radio shut off

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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