It is probably not just “mounts” it the the threaded parts that are in the unibody that hold the entire rear subframe to the car. These points crack and rip out of the body. The only way to fix them to remove the entire rear end, exhaust, gas tank and then buy reinforcement plates and have them welded to the unibody.
Search the BMW forums for more information and see the link.
The leak/ wetness was further into the car that the door seal. It was literally running a stream of water onto the floor.
I am not trying to pick on you or anything like that, but would like to let you know what you are getting into.