Electrical window problem

Hi all,

I have a small problem with the electrical windows on my 450sl. When I close the door it makes a trembling sound on both doors. So I removed the panel and good see that the window is gliding up and down in a metal U. The windows is smaller than the metal U and a metal S shaped thing should be on stuck on the window and glide over the metal U. On my car the S shape thing was on the bottom of the door.
Now the question, how does this metal S thing get stuck on the window? With glue?

Thanks in advance for any help.

Jeroen