I suspect that they disconnected the battery before the MKIII navigation computer fully shut down. That killed your nav computer, and the car's video module took over control of the display. if I am right, then you will need a new navigation computer.

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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