Most repair shops are VERY careful when it comes to attempting to repair anything safety related, such as a seatbelt retractor. MB doesn't publish anything that shows a repair procedure, they recommend replacement only in case of a failure. You probably won't have much luck with attempting to repair it yourself, it's just not meant to be tampered with.
You can click on the "Fastlane" or "PartShop" icons on the top of the MercedesShop screen and see if they sell the belt reels or not.
The trim by the seatbelt it pulled outwards at the top, then it is pulled forward and up to disengage from the hat shelf. It can be tricky to engage it back into the hatshelf to reinstall the panel. The bottom seat cushion is easily removed if you want to remove the belt assembly. Once it's unbolted from the floor (under the rear seat) the belt anchor and webbing can be fed through the slot in the interior panel trim, then the interior panel can be set aside. The reel assembly can then be removed.
I don't believe the rear seat belts on this one are SRS/ETR(emergency tensioning retractors), so makes it a little easier than the later cars.