Some things to consider when replacing the trunk seal, something I have done on a number of different MB models:
1.) ALWAYS use a OE brand seal. Despite the idea that "anyone can make a seal", it is a highly engineered part that has all sorts of little details on it to make absolutely sure it works. The quality of the material is critical as well. How long did the one that's on there last? Yeah, now you know what I'm talking about.
2.) Make sure the seal channel on the body is surgically clean. Use a dull popsicle stick or other piece of flat wood, and with the channel flooded with 3M Adhesive Remover, scrape until every last bit of the old seal and glue are gone. I dam up the ends of the groove with some putty or a rag so the 3M Adhesive Remover will stay for a bit. You'll probably have to pour some in, let it soak for a few minutes, then rinse, lather and repeat, so to speak.
3.) Put an old sheet or tape masking paper over the back window area down to where the trunk opening begins. Why? The seal is going to have to be placed over the trunk lid AFTER the adhesive is applied. This prevents you from getting adhesive on the window and painted surfaces around it.
4.) Use 3M black weatherstripping adhesive, NOT the yellow stuff. You have to apply it to both surfaces, so lay out some towels or paper on the floor next to the car to put the seal out. It has some curves in it that will make it twist, but don't worry about this. Put a thin layer of sealant on the bottom edge (the bottom of the "U", if that makes sense) of the seal and a similar amount on the bottom of the seal channel on the car. Don't put it on the sides of the seal or channel.
5.) How the heck do I know how to put this thing on the car???
Before you start, look at the seal closely. Under the lip in one location you'll see the Mercedes part number molded into the seal. This goes at the exact center of the trunk opening on the trailing edge, that is, just above the license plate or trunk lock cylinder. Pay no attention to where the seams are in the seal. It's not relevant to the installation - the part number IS. The "C" shape of the seal should have the opening of the "C" always facing out.
After the adhesive is in place and has tacked up, start at the center above the license plate where the part number goes and gradually work around, a little on each side at a time. DO NOT stretch the seal as you put it in place. As you get closer to the top center you'll have a good feel for the tension on the seal. You shouldn't end up with any excess or a "crimp" by the time you get there, or you have stretched it.
6.) Once the seal is in place, go around with your fingers and apply pressure to it all the way around the trunk opening. Make sure it's sitting upright in the channel and not cocked or angled. Once you're satisfied that it's in place, close the trunk lid and leave it that way overnight.
Dan