I have noticed when looking at my rear window seal, which by the way needs replacement as well that a slight amount of sealant was used in the original installation in the grooved area which fits the glass. I can see where that makes sense seeing as the rear seal on these MB's aren't the best design if you have four seasons. If you look at several American designs that are similar they use rubber fingers that positive lock against the window during installation inside the glass groove to keep water from penetrating. Any flat surface that is against another flat surface is bound to fail unless you fill the void that will contour to the exact surface between the two. I'd use sealant in the groove as a bottom line answer.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan