All 03 and 04 cars have the MOST bus. My 03 S came with just the radio and 2 speakers in the dash and it has the MOST bus. There is some information in
http://986forum.com/forums/boxster-general-discussions/67216-people-have-swapped-their-cdr23-fibre-optic.html that might help. I don't think anyone here tried the $99.99 adaptor available on AliExpress that Nine8Six mentioned in that thread.