I personally would assume we always assign primary responsibility to the owner/operator. It is their responsibility when buying/operating to do their due diligence on is it safe/legal... If the manufacture sells the vehicle under false pretenses then the person/company with the business contact should settle with the injured party, then seek compensation as needed. Of course if the owner cannot cover the damages, then sure bring the manufacture in directly.
I still think the most difficult part is going to be on expected minimal reactions. Very few auto accidents have a single contributing factor, so when a road isn't properly maintained, sun is interrupting optical sensors, a truck stops in a intersection, and autopilot drives into the side of the semi without trying to avoid the accident. The primary responsibility will still be the truck driver, as they started the chain reaction with a illegal move.
Of course the manufacture will be expected to take corrective actions, or risk direct action against them to force it. Regulations will need to evolve to apply this pressure, to keep a constantly improving minimum standard of safety.