I think they will have to use base 6 arithmetic for these head units...
My friend has a VW Passat with an 8 disc changer. His head unit (factory) only controls 6 discs, so he has to go to disc 6, then do {next disc} to get to 7, etc.
I know many players don't even have a {next disc} feature.
-Trevor
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Mk 2, Green 12GB 080000349