Welcome to the group! The e60 is a enviremental wonder, it will start recycling while you drive it. Not really.... But the Germans, with their green laws, have made it hard on the auto industry there.
I have replaced the rear window trim piece twice, June of 08 and August of 14, first was under new car warrenty. The window will not leak, trim is for looks. The trim piece across the top of the windshield is another piece for esthetics. The windshield will not leak as the trim degrades. The difference is, the windshield trim can only be swapped out when the windshield is changed out. Or at least that is what a reputable auto glass shop told me when I approached them with the replacement trim in hand. Told me I wouldn't be happy with the job if they were to try and just change it out.
Now the cowl piece a the bottom of the windshield will also degrade to the point it must be changes out, August of 14. That's about a 1.5 hr project, with the wiper arms removal and reinstalation taking about an hour of the 1.5 time block.
Just change out the rear trim piece and move on, about 45 minute job and $45 for the trim piece as memory has it.
Good luck.