The electric sun roofs are a PITA. If it won`t fully retract, in the trunk on the drivers side behind the trunk liner is the sun roof motor. there is a nut to turn by hand to open or close. I think counter clock wise is to close it.
the side felts can be replaced w/o removing the roof. there are 3 or 4 phillip screws under the edge that hold them in. the front rubber seal is the real humdinger to replace. if you do, pay real close attention on how it fits into the groove when you pull it out. I used some spray silicone on the rubber to help it slide in. start in the center and work out from there.
I think the W123 & W126 sun roofs are the same.
If you do decide to remove the roof, slide it to the rear 1/2 way. along the front edge of the liner pull down,& the liner has some clips that pop out. think there are 6. slide it forward and up through the opening.
www.pauldrayton.com/uploadfiles/merc/service/library.html
there might be some help here.
Charlie