Nope, a car is not a system that was built to be 'sold', it was built to transport people/things safely. If you remove a part of a system and the system still w

Nope, a car is not a system that was built to be 'sold', it was built to transport people/things safely. If you remove a part of a system and the system still works as intended, it was not part of the system. The head-rest can be removed and the car will still run but with less safety. Hence: all parts of the system are equally required to interact and make the system do what the system should do.