This topic has been addressed in multiple threads. For many, the clips that hold the trim above the window leak water and many have solved it by pulling the trim and sealing the clips with silicone. There may be a few other causes as well which are also discussed at length. Do a search for "wet seatbelt" or "wet seat belt" and you should find all you need.
Good luck and keep us posted.
Good luck and keep us posted.
