Here's an update to my radio control mod that I've started a few years ago. Shrunk everything down to transistors, instead of relays, and opened up more potential to use different communication protocols.

 

Do you plan to release the diagram and code into a github? Looks promising, I was planning to build a central computer system for s2k and you are kind doing the first steps for this. What I had in my mind was replacing the head unit with a raspberry pi+touch screen and connect everything with an esp32 to the raspberry pi. I mean, imagine if you can see live obd data and control all the switches from a single unit. I am not sure about how hard it would be to connect the sound system to the pi but I don't think its impossible.