1993 300sd. The driver's side seat belt won't retract, need to know how to open the car up to get at it, and where to find replacement if I can't fix it.
Unless you are the sort of guy that fixes light bulbs, don't even think about fixing the inside of an inertia reel seat belt. Buy the new part and bolt it in (removing and refitting trim pieces). The inertia reels are front/rear and left/right specific.
You can replace the rear ones, the front ones are hooked to the SRS system for the crash tension charge. Replacing those requires you to reset the SRS system.