This Adaptor is available to do some of what I would like to do with my project.

It connects to the head unit via the changer port too:

A quick google search shows the following Pin out for the back of my head unit, port C being the D Changer port:

I'm assuming the cable I currently use makes use of a ground pin and two of the low level input pins.

It's good to see a 12v pin on the changer port - with a regulator I'll be able to power my pi without needing to route anything on the front, meaning I can have a nice clean install!

There's also some Bus pins and some Remote control pins. Some investigation will be needed to see what i can do with those!