The part that broke off is part of the clip and the clip removes from the windstop so that you can replace just the clip. You need a new clip for that side. At my dealer is is about $3. Buy the replacement clip and then you will see how it is attached to the windstop. The clip just prys off the windstop and the new clip snaps on.
http://www.renntech.org/forums/index.php?showtopic=2764&hl=windstop+clips