I think in issues of public safety like that, it would be more a matter of law and not digital rights. Tinting your windows too dark, cutting a brake line, or adding distracting lights to the exterior are all ways to modify your car that are illegal (where I live, at least), so there's really no need to have other protective measures against that built into the car. I'd suspect driving software modifications would be treated similarly, with DRM or some similar system mainly being used to detect the illegal modifications as opposed to trying too hard to prevent them.