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.