1986 Pontiac Parisianne-Driver side front door window off track

[pdn1000]

I got into my 1986 Pontiac Parisianne yesterday and when I tried to roll up the electric window, it tilted forward, off its' track I'm guessing, exposing a hole in the window, toward the back, which I think must be for some kind of retainer clip that has worn out or at least come off causing the problem.
Does anyone know what this missing part that I'm calling a "retainer clip" is really called ? And where I can get it ? And can anyone tell me how to install it ?
Thanks in advance for any help you can give me.