The locks run off vacuum that is produced by the vacuum pump on the engine. There is a black "egg crate" looking resevoir in the trunk under the rear shelf.
You have a leak in the system if the locks dont work. A leak anywhere in the system can cause havock in the rest of the system.
Check here for troubleshooting basics.
http://dieselgiant.com/mercedesvacuumtroubleshooting.htm