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Join Date: Jan 2011
Location: United States
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Upgrading to the Mk. III or IV is plug-n-play. If you do have to get a new nav computer, you may as well upgrade. I'd go with the Mk. IV as it was the latest nav computer for the E39; not to mention offering a 3D map view among other features. I'm not sure if you can use the 4:3 screen with the Mk. III or IV computers, so you may have to upgrade to the 16:9 widescreen monitor also.
The P/O of my 2000 (11/99) Beast did this uprade (Mk. III and 16:9 monitor) and the only problem I had was that for some reason, the original radio module (C43) would not allow me to access certain radio functions like RDS or PTY. When I swapped in a BM53 (the radio module BMW began using with Mk. III), all was good.
That was my experience. Some have said they did not have any issues with their C43 module when they upgraded to a Mk. III.
But, outside of all that, again yes, the nav swap to Mk. III or IV is plug-n-play. Just remember to unplug the trombull as it is not needed with the Mk. III and IV; in the Mk. I and II cars, the trombull is seperate from the nav computer. I guess the newer nav computers have this feature built in so the one in the car is not needed, or so I have read here on this board.