1991 Chevy S-10

Not sure, might be a plastic keeper that broke, backside vertical of window. Helps to keep window in proper position, I can't be positive about your window, I have seen it in other GM vehicles. I bought some of those keepers at a national brand autostore. If the window leans forward enough, can you see a hole or any place a keeper would go? It's been years since I did kind of repair, may not even apply to your vehicle? It was just a thought. You may have to remove door panel, see if anything broke down low.