the nav computer is located in the trunk area, on the left side, behind a cover door. you open the trim cover by pulling the handle, and you should see a device that can take either a CD or a DVD (if the letters DVD ROM are printed on the front fascia of the module). that device is a BMW Navigation computer, that device creates and presents the visual menu on the display that is in front of the car. Earlier models E53 would have a CD based module while later models would have a DVD based unit, both units are interchangeable, obviously the DVD unit is more advanced than the CD model.
one of the previous owners may have replaced the original navigation computer with a different unit. The nav computers are known to have issues ranging from start up failures to map disk reading failures to failures to receive and hold satellite locks...
Your monitor flickering might be a sign that the navigation computer fails to properly boot, and as a result, fails to display any information. That issue could be linked to your radio issue, or could be a separate issue...
Check for loose connectors, corroded connections, and water damage - water likes to collect in side pockets below the navigation computer on the left and bank of the fuses on the right. Another favorite place for the water to accumulate is the tub where the battery is located - the water can get in to those areas from a clogges drains for the sunroof - the sunroof has 4 drains, 2 at the front that go from the front sunroof corners to the bottom of the car in the little area between the front wheelwell and the front door (A pillar), and the rear 2 drains are draining at the top of the trunk opening - once you open the top hatch, you can see two holes next to the hinges (the holes might be open or closed by a thin rubber flap). this is a concern if you have moisture or moisture damage in the trunk.
the nav computer is supposed to have a little red light that should blink during start up process, reading of the disk, and otherwise should stay steady on... Is the little red LED on?
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