I wonder if the 'WIDESCREEN_MODI' byte at the end is what controls the ability to do split screen mode while in Radio mode. I've seen a couple of retrofits of the Mk III where this functionality is lost, although split screen works fine with the map and directions, you can't get radio/CD and directions. With the MK IV turning the radio on and off at the right time while changing the firmware can cure the problem, but with the MK III this does not work.
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