check the fuse located in the trunk ,right compartament ( E53).....though not the fuse is the problem but it helps .I have the same problem but when I need to see the nav screen I unplug the fuse and then 1,2 sec put it back (engine idle)...it works ...life is back on the nav's screen.I dont know what's the deal but I'm not taking the car to the dealership cause they r idiots (VOB Rockville MD) and they will ask me to change the entire dash for sure . Let me know if u find out what is the cause