There will never unfortunately be an open-source car. The nearest you will get is by getting a well-documented one with good OEM parts availability and converting it to an EV which eliminates a lot of the proprietary computers that seem to cause issues. EV drivetrain and battery parts are mostly well documented and interchangeable and can even be open-source, and Air bag and ABS computers in older cars also mostly work stand-alone (ie they will work properly if the cars computer is not there)
Newer cars have much better safety systems but have unfortunately gone all in for making things difficult.
Well, at least here in Brazil, vehicle's manuals were far way better, even explaining the process of building the car. This reminds me of the history of a guy that rebuilded a Volkswagen Saveiro just reading the manual: https://youtu.be/ZKe1gl4WCvc?si=nIvKtAESdmn03bCn (Brazilian Portuguese, with auto generated subtitles)
It depends what you mean by "open-source". You can build the entire drivetrain of the equivalent of a stereotypical mid-century domestic car with no parts from the original manufacturer, for example.
Newer cars have much better safety systems but have unfortunately gone all in for making things difficult.