Not sure if your 190 is like my 87 300E, but I fixed the sunroof myself with the same type of problem. If you pull the sunroof door cover off (not the headliner) you can clearly see the sunroof cable, a portion of the track and the tilting mechanism. For me the cable poped out of the track and frayed and bent to where it would not provide pressure for closing. It is super easy to replace the cable, taking all of 5 minutes, but if your track is not functioning well then the cable will keep on getting messed up. Replacing the track requires removing the headliner and the rear window I believe.

Once again this is for the 300E, not sure if yours is the same as mine. But you should at least take a look up there to see what the problem is.

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2007 Corvette Convertible Torch Red
1996 MBz SL500
2008 Harley Davidson Night Rod Special