sliding door small problem

We have a '94 Caravan, and while shutting the sliding door, a long
piece of black plastic fell out. I initially thought this piece is supposed to fit under the roller that the door slides on; but there is still another piece
under the roller, and the door still shuts fine...So now I'm scratching my head...Any suggestions.

Did you look under the sliding door? If you look hard, you should see where it came out of. Leave the piece in the car. Next time you go to a garage ask them where it goes. Good luck.