Thanks for that idea Hugo. It should be very similar since Honda seem to use only very slightly modified Alpine gear. They swap two pins in the cable generally.

I will have a look. It is very 4 bit oriented alright. Everything is done in BCD basically. I think it might only be a 4 bit checksum. It's the algorithm that's got me at the moment.

Did you see only ever 4 different bytes transmitted? 0x10, 0x70, 0x13, 0x73? They seem to translate into 00, 01, 10, 11 (in that order)