Actually sniffed the bus today, 1200 baud serial seems to give proper data. Just rx/tx crammed into same line with some address comm protocol.
Cheating it to think there is changer connected and looking for the next/prev etc command might be possible with some time sacrificed, actually sending proper track time data looks like too much of a bother.
I think I might just use the AUX method and just wire the steeringwheel remote to the unit separately. Much less time consuming.