That sounds like a dead nav computer. If you have a blank screen, then you have one of two problems: either the screen is dead or the nav computer is dead. Since you have the luxury of a spare screen to test, you most likely eliminated the screen as the problem. Therefore, the problem is the nav computer.

__________________
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