You have a problem that is actually common with our cars. I have owned my car eight years now and bought it new. I had a similar problem twice. In my case the ac was off but it would turn on and off on its own. Now matter what speed setting I pick it always blew at the same speed. Not sure if it is a relay or the actual switch unit in the car. I had it replaced under warranty twice and never paid attention to the dealership explanation. I think it is an 800 dollar part though which I am sure you could get cheaper directly from Mercedes.