The Germans like to use what is called a final stage unit to control electric motors in certain applications such as auxiliary fans and blower fans. It is a potted device that uses semi-conductors and integrated circuits to control the speed of the fan. It is almost always located on the fan assembly itself and usually requires replacement of the entire fan assembly as they often do not sell them as a separate unit. I hope this is helpful to you and if you need more specific data, then I will request our resident MBZ expert to assist you. Dr. Hagerty?