when transistors were young, car radios had a device made with two metal plates and separated by a mica washer. no, it wasn't a capacitor, it was called a spark plate. it was to keep the voltage spikes from causing damage inside the radio. it was usually soldered to the frame of the radio and the incoming +12V wire was soldered to the other side. now they use zeners. the voltage spikes tended to be higher because car voltage regulators were made using relays, not transistors. relay coils in a car voltage regulator could punch out inductive spikes of 100V or more. the alternator or generator field coil could produce some high voltage spikes when the relay contacts opened as well. these days it's done with transistors, and the field coil is linearly controlled.