Or the clip that holds the plastic guide has fallen off. If so look for a channel shaped black piece of rubberized plastic at the bottom of your door well once you have the door panel off. If you do not know how to take the door panel off ask before you try. Otherwise you will almost certainly break it.
[Edited by ejsharp on 04-27-2001 at 11:01 PM]
1993 190E 2.3
2000 Toyota 4x4 Tundra