The best way to do this is to use a standard physical interface for the storage. Then to do a factory reset you remove the storage device, plug it into any PC,

The best way to do this is to use a standard physical interface for the storage. Then to do a factory reset you remove the storage device, plug it into any PC, nuke whatever is there and replace it with the latest clean image from the manufacturer's website. This also makes the vehicle almost impossible to brick through damage to system data (worst case: remove drive and reimage) and allows the new owner to do the same thing to be assured that the vehicle is not harboring malware.