I believe it is vacuum controlled, engine must be running in mine for it to work. When I bought my used and abused beast, I thought mine was non functional as well. One day, I held if for almost 20 seconds and it suddenly worked. Now I use it periodically, whether I need to or not.
Try wiggling them for and aft a bit as well.
1986 300E Anthracite + ECodes + MB Mileage Award