Does anybody know of any software that implements Android Auto or Apple Car protocols to display content on existing head unit? Maybe something that could run on a Raspberry Pi.
You could try looking into some of the Android forums where they have unlocked the head units and have demonstrated being able to flash custom roms, although may be a bit different from what you're looking.
That does seem interesting. I was looking at whether it was possibly to flash/replace the head unit software if a few years old Camry the other week, but didn't find anything useful. Do you happen to have info on which android forums someone should look in?
This is what I'm actually interested in.
Here's a thread from XDA about the Honda pilot, may be a good place to start perusing