Switches on and off very quickly? Doubt it. They're normally an array of high wattage resistor to drop the voltage to the motor to decrease speed - the motor runs flat out for the the given volts it gets. Even if the resistor pack has failed most designs will allow full voltage to the motor via the full speed setting........of course, Citroen could be different. Most designs incorporate a fuse, often a board mounted component, which means that the whole pack has to be changed.