The auto manufacturer, as they're the entity actually operating the vehicle. You can absolve yourself of responsibility with enough legalese "self-driving doesn't work, it will kill your kitten, you're responsible when you plow though a Denny's" but it doesn't make it right. Mercedes is taking liability, it's possible.

You have no control over how your car's self-driving operates, you can take over control but that doesn't mean you're driving. You're the passenger in one of those student-driver cars, you might have your own steering wheel and brake but it doesn't make you the driver. The life-guard isn't the swimmer.