The PDC sensor housing is two-fold: the exterior cover and the interior sensor tube. The tube and exterior cover snap together tightly against the hole in the bumper. Check out pages 13 and 14 and look at the inset picture of the mounting where part 1 and 3 meet. FYI, this will be very difficult to do without removing the bumper cover.

http://www.xoutpost.com/pdf/e53/BMW%2...%200007032.pdf