Well, I hit it hard today and finally worked out how to enabled VIM!
I've lost count of how many times I've flashed it but I got there eventually. Tomorrow I'll hopefully install the media player in the glovebox.
Next step for me will be to start learning GMLAN with the goal of capturing the steering wheel control packets and performing manipulation before outputting them to the media player.
Thanks to everyone who has contributed to the wealth of information on this site, I don't think I would have even attempted any of this if it weren't for the documentation that got me started.