To learn more about diagnosing Mercedes problems go here: https://mercedessource.com/problems
Kent shows a good example of the reason you want to do a complete and proper diagnosis of the problem BEFORE you order parts. It can be very frustration to get the car all torn apart only to find you order the wrong stuff. This is not just Mercedes related. It applies to working on any car! Sometimes it may require opening up some section of the car but it will worth it in the long run in both time and frustration saved.