The easiest way to find the problem is to check the operation of the vacuum valve located under the rear seat on the left side. Check to see if you are getting vacuum to the valve (engine has to be running). If you are getting vacuum then you need to see if the switch on the dash is sending voltage to the valve when the switch is operated. If you are getting vacuum and a signal from the switch then the valve is bad.

__________________
Mercedes Benz Master Tech - Retired