I think your referring to the inlet neck on the radiator. This seems to be a problem on all MB's, and it also happened on my W124. I think theres a place who replaces the plastic housing around the radiator--other than that, your looking to get a new radiator. I dont know of the place that does this, but look on the archives. Lots of posts on inlet necks, and radiators.
1989 300E (108k miles)