I don't think it is that unrealistic. It seems that for 1), aftermarket radios are available for most cars and can utilize the speakers. There are various adapt

I don't think it is that unrealistic. It seems that for 1), aftermarket radios are available for most cars and can utilize the speakers. There are various adapters available. For 2) I can imagine HDMI or DVI, if necessary with a more sturdy connector. Stock monitors have supported multiple resolutions for 20 years. For 3), it seems there is information available to access this from CAN or LIN buses or ODB. Lots of hobbyists do this. I can imagine you could buy an adapter for, say, "Ford 2010-2014", like you can do with various aftermarket parts. It might be a niche market, but so are all kinds of aftermarket mods for cars. And I think if somebody did to car stereos what Nest did to thermostats, it would be phenomenal.