Your switch is out or your blower motor is on its way out. Mercedes are difficult cars to work on unlike the BMW and VW. Those cars are made to work on yourself. If I were in your shoes I would take it to a dealer and they can tell you exactly what's wrong with it. Depending on your financial situation is. I have a friend that's a Mercedes technician and I just forwarded your question to him. I will send you the info when he replies.