My 86' 300E has the same problem,
I tried hot glue it didnt work that well,
I tried 3M Spray adhesive and used spring clamps until it dryed, that worked a little better but still not perfect, it did last longer than the hot glue. I was going to consider some powder furniture glue but I'm affraid any glue that sets-up hard will fail. I saw a similar post on the Ritt-easley list but responses were to try 3M.
I have seen different types of 3M adhisive, one in particular is used for headliners it will withstand the heat generated from the sun, the standard stuff will come loose.
I'll watch this thread and see if anyone comes up with anything better.