530i 2001 Sat nav problem

Hi folks, have a problem with the sat nav on this UK E39.
Turn the key to position 1, the monitor comes on and everything seems to work. As soon as i start the engine, the display goes out just with the
occasional flicker.
This is happening all the time. I have disconnected the battery and left for 30 mins then reconnected but the behaviour is the same.
Appreciate any suggestions.    
--

jeremy

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