To my understanding PE stamps are not normally used for machinery design or automotive design. Those designs go through a huge number of regulatory hurdles and certification processes (crash test, MPG, fire control, UI layout, handling and roll over, electrical breakers, etc etc etc) that take the place of the PE stamp. If you were going to require PE stamps for an automotive design you would end up with thirty or forty stamps for each part of the design - engine, transmission, suspension, drive train below the transmission, electrical, outer shell, interior layout, crash management, pollution control, specialty sub systems within each of the primary systems. The company that is designing, assembling and selling the finished product is liable for all of the certifications and regulations that are required to put a car on the road.
The reason why there are so many certifications and regulations around a mass manufactured item (cars, toys, appliances, garden tools) is the sheer number of them put on the market. Because someone is producing 500,000 units of a given thing means that more oversight is required because more people could be injured if there is a mistake or oversight in design.
Most cases I see where a stamp is required: The item being designed is a one-off or custom job. You need someone who has the authority to make the call that something is safe and will perform as advertised.
Hell, my PE application (still applying, not approved to take the exam yet) doesn't list Automotive as an option. Maybe you could file care design as an Industrial PE item. Everything else doesn't fit (Civil, Nuclear, Aero, Mining, Architecture, Chemical, Petroleum, Fire Protection, Electrical, Metaullurgy, HVAC, or Naval).