Is this after the navigation retrofit? It sounds like your radio module is not connected. Try verifying that you are getting power at your radio module.

Also (this may seem basic, but I want to cover the diagnostics basics), you turn on the radio by pushing the left knob on the 16:9 display.

Do you have DSP? If so, do you get the "DSP" menu option on the main menu of the nav screen? If not, verify that you have power at the DSP amp.

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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