I have a 1995 E320 Wagon (only 42k miles!) and it had something sort of like your problem happen. The sunroof always tilted up, but would sometimes open a little, then stop, and if you played with the switch it would finally open all the way. It had the same problem closing. I got it all the way open and lubed every piece of shiny metal I could find, and now it works flawlessly. I don't know if the sunroof switch in your 190 is the same or not, but in mine it is a 4-way switch. It slides forward and back for the actual open / close operation and the switch goes up and down for the tilt / close function. Try playing with the switch. Another thing to try is stick the key in the driver's door with the car all closed and turn the key to the unlock position and hold it for 5 seconds or so. This should open all the windows in the car and the sunroof automatically. They should also all close if you lock the car. Maybe your 190 does this, I don't know.
Here are some other minor things to look for: Does the interior light come on in yours when you open the DRIVER'S door? If it doesn't then you will have to change a $60 relay. (Mine is bad) See if the alarm works correctly as well: it should arm when you lock, disarm when you unlock. Mine only works from the tailgate. Just out of curiosity, does yours have heated seats, memory seats, and a power steering column? How about wipers on the headlights? Please fill me in.