It is a big problem, only the dealer can fix it. The rear interior has to be completely removed (I mean everything) and then the entire assembley must be replaced as well as the vacuum plug. I think it will cost about 400-500 dollars. This scenario happened to my S420 but I was under warranty. All I heard was a pfisst sound coming from the rear with no movement so I took it to the dealer and for two weeks they were fixing it. While they were in there, they also broke the heard rest retractor, so they had to replace it. Don't take it to anyone except the dealer or certified tech because they will fool you by sealing up the vacuum leak with some Epoxy and it will eventually leak again.