The same grinding was happening on my recently acquired 1894 190d. Problem was a stripped nylon gear. I found a replacement gear at PicknPull for about five bucks. To install it, I...
1) Removed the left trunk (boot) liner.
2) Removed the two screws holding the antenna to the body.
3) Loosened the collar at the bottom of the antenna mast.
4) Pulled off the cover of the sunroof manual operating nut.
5) Removed the nut under the cover.
6) Removed the three bolts holding the sunroof motor in place.
7) Rotated the sunroof motor to remove the gear cover at the back.
8) Pulled out the plastic gear.
9) Lubed the new gear with plastic gear grease (got grease on line)
10) Re-assembled everything up to the point of replacing the cover of the manual operating nut. This nut adjusts the clutch tension.
11) Using trial and error, gave it enough clutch tension to operate the roof without stripping the gears.
12) After reading lots of discussion re-rail lube, I went the cheap route and used "SiliKroil" spray on a rag.