The most common W463 problem is a sag in the backer just behind the sunroof.
This is caused by the double stick tape that holds the mushroom-head Velcro (what is that stuff really called) coming loose. Getting in there successfully to install new double stick tape is unlikely. I think that construction adhesive and clamping would hold it nicely. If the headliner needs removal later it shouldn't be too hard to cut the adhesive.