[simulator5g]

This isn't actually true. You just need a module to interface with the car in place of the original head unit. For example the iDatalink Maestro can connect HVAC controls, engine diagnostics, tire pressure, etc to compatible head units. And there are a lot of compatible head units, basically all the worthwhile ones have Maestro support.

[CamperBob2]

That's good to hear, I wasn't familiar with Maestro at all. But it doesn't do anything to address the proliferation in form factors. I can get nice aftermarket head units from a couple of different Chinese vendors for one of my cars, but they had to be specifically designed for that exact application. I ended up adding wireless CarPlay to that car's original head unit with an ugly internal/external adapter solution. It works well enough, but it was a pain to install. Hopefully similar hardware can be made available to GM customers in the future.

For the other car, though... no way, no how. It is stuck with what it has. Its touchscreen is a nonstandard size, seamlessly integrated into the dashboard, and not used by any other cars AFAIK. Fortunately it's also pretty good. It supported wireless CarPlay from day one, and it isn't from a manufacturer that relies on fracking its own customer base for $20/month. Safe to say nobody will be building an aftermarket replacement for it.

[simulator5g]

You just need a dash kit, which someone probably makes for your specific make/model. If not you could 3d print something. Any good stereo shop can help you out too. Its kind of hard to find a good one though.