I made some more progress today. I was able to able to transmit the left joystick message and programmatically increase and decrease the volume on the Radio from my custom module. It was a basic program just to prove that I could indeed change the volume by code without touching the controls. I then logged over 7000 lines of messages and hope to do some decoding of MPH, RPM, etc... For this first mod I have everything I need except the MPH.
I work overseas a month on, month off... I head out to work tomorrow so I'll not have access to the MC for awhile. But I do have the log data and hopefully I'll have a chance to look at that on my spare time.
I'm sad to say I could not locate the NAV prompt volume (not sure it's even exposed on the Bus)... It could be that the NAV functions are all "with-in" the NAV unit and not available on the network. That's too bad if it's true. I was really hoping to be able to sync my music and NAV prompt volumes... Maybe I'll get lucky and find it if I keep looking.