It could be a problem with the screen itself or the navigation computer.

Does the nav drive have a red light that comes on when you first start the vehicle? If not then it could be a fuse or a bad nav drive. If yes then I would try to find someone near me that has a Mk3 or Mk4 nav drive and swap them to see if that is the problem. Through a process of elimination you should be able to narrow down the cause.

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