Automotive voltage regulators "sort of PWM" the field current of the alternator. The reason I say "sort of" is because the VR is nothing but a bang-bang "thermostat like" system: Turn off the field switch (usually a power transistor) if the bus (battery) voltage is > set point; turn on the field current if the bus (battery) voltage is < set point. It is the inductance (several H) of the field winding that creates the appearance of PWM.
If you hang a scope probe on the field connection to the alternator, you see a low frequency pulse signal that wanders between 25 to 100Hz, where the duty cycle of the ratio of on to off determines the average output current. Just like your house where the temperature is always increasing or decreasing, the bus voltage (and field current) is always increasing or decreasing around the set point.