Most cars are "proprietary". The only cars that come close to being open is older vehicles that are no-longer manufacturer with a good after market parts market

Most cars are "proprietary". The only cars that come close to being open is older vehicles that are no-longer manufacturer with a good after market parts market. e.g. I bought a Land Rover Defender because it is the closest thing to an "open source" vehicle. However you are giving up many modern conveniences and you will be doing a lot of work yourself.