I don't know if they are still at the ford dealers, but they are very easy to get. mustangs plus, mustangs unlimited, dallas mustang, National parts depot, california mustang, etc will all have them. With the chunk you are talking about I would be willing to gamble thats what it is. Pull the panel off and take a look. you will have to remove the back seat and then like 4 screws. crank comes off with an allen wrench.
I did take off those panels and found everytime I would crank the window up or down the plastic thing would pop out of the track. While I was there I also applied Lithium grease. The tracks look ed like they have never been greased.