I don't think you have any other options other than changing the nav screen to a MKIV screen or wearing non-polorized sun glasses.

My friend with a 2001 M5 has the same problem with his MKIII Nav screen. I do not have this issue on my 03.5 DVD Based MKIV Nav screen or on my 05 X5 w/ the MKIV nav screen.

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'05 Le Mans Blue X5 4.8is & '11.5 Interlagos Blue E92 M3