The whole procedure includes taking off the panel, but then re-installing it can be a pain without the device that preloads it into the front gasket. I think if you just lightly grease the bottom sliding surface of the rail, that should get you most of the way there. The disadvantage of doing it this way is it's near impossible to grease the tracks that go into the back of the roof.
Whatever you do, make sure you clean the rails first and use the MB slide paste.
2000 C 2 3 0 Sport :: 180k