I have basically worked out the baud rate, how the bits are encoded etc. I have a service manual for my headunit so I should be able to emulate the circuitry on that fairly easily.

My biggest stumbling block at the moment is how the checksums are calculated. If I send you some sample packets and point out what certain things are - is that something you think you could tackle? Do you know about stuff XOR, CRC etc?

Currently all this info is basically in my head (and a dodgy debug program I have written). I haven't really written/typed anything out yet...