how do I replace the locking check-back fuse and the sunroof fuse?

Hello, Test the sunroof switch. It may only work in the one position because the contacts for the other portion of the switch are not transferring signal to the sunroof motor. Fuses and fuse locations are listed in the owners manual (or should be), but you will have better luck consulting an online guide to locating the proper fuse panel and fuse. That said, I don't thing you have a fuse issue. If you can lock and unlock the doors electronically from another switch, the fuses for the door lock actuators are good. Mercedes Benz are great about complicated electrical systems, so I would advise using a repair service for that one.