Although Mercedes has parts that have part numbers on them, those parts may only be sold attached to another part and sold as a complete unit.
For example, when I tried to order a speedometer face plate, they said that they couldn't find the part number in the system and then ended up telling me that I have to buy the whole speedometer assembly.
The part number I gave them was the one I read off the face plate:
I believe in your case, that you don't have to buy the sunroof Motor with Gearbox. Sometimes, the part number on the actual unit is different than the part number that is on the box.
For example, a Mercedes sunroof motor & gear part number for a W201 (1987-1993) is 000-820-32-07. The box says A 201 780 02 07 .
and
Mercedes part number 000-820-32-07 is a sunroof motor gear good for some W123 and W116 chassis cars. If you look closely, you can see the part number on the unit is different.
Here are some of them for sale on ebay:
http://search.ebay.com/mercedes-sunroof-motor_Parts-Accessories_W0QQsofocusZbsQQsbrftogZ1QQcatrefZC12QQfnuZ1QQfromZR10QQsacatZ6028QQcatrefZC6QQsargnZ-1QQsaslcZ2QQsadisZ200QQfposZQ5AIPQ2FPostalQQftrtZ1QQftrvZ1QQfsopZ1QQfsooZ1QQcoactionZcompareQQcopage numZ1QQcoentrypageZsearch