Normally there is an adjustment bolt with a jam nut usually located on the top of the steering gear box. I haven't looked at this yet but will and if I locate it I will post a picture for you. I will be gone for the weekend but will try to get back to you the first of the week.
But then again all my past experience is with American cars, once I look at Mercedes set up I may be just as lost as you.
If I figure anything out I'll be sure to let you know.