I really REALLY wanted a head unit that supported Android Auto. I also wanted a head unit that was open source. These things appear to be mutually exclusive, unfortunately. So, instead, I have bought a Kenwood DMX8018S, which has three camera inputs.
They are going to be used for Reversing (obviously), a Front Camera so I can make sure I don’t rip the front spoiler off when parking, and Raspberry-Pi-Input. Unfortunately, the inputs are just Composite, not HDMI or anything sensible, but at least it’s a start.
The other thing that makes it attractive is that it can remap the existing steering wheel controls to internal functions. For example, the steering wheel has a Mute button. I will NEVER use that, so I will remap that to the ‘OK Google’ button.