Tube car radios used a vibrator, which basically starts and stops the DC current repeatedly. Old tube radios (and guitar amps) operate off alternating current. They use a power transformer to step the voltage up from line voltage (110 back in the day) to typical values from 300 - 500 volts. The transformer works with AC current, because each time the current reverses direction the magnetic field around the iron core collapses, inducing a voltage in the secondary coil. Well, cars operate on direct current, so the current is not reversing direction, so there is no collapse of the magnetic field, and no voltage can be induced. So they added a vibrator to the primary side of the circuit. The vibrator interrupts the DC current repeatedly, which results in a collapse of the magnetic field in the primary coil and induced voltage in the secondary coil. The buffer capacitor is used around the vibrator, and needs to be 1000 - 1600 volts capacity because, yes, the voltage seen is that high, due to the flyback voltage I described above. Even when replacing an old paper cap with a film cap you still need to use a high voltage capacitor in that application.