Crawl down under the area with a flashlight and look up for a panel in the area with hole a little smaller than size of the fins. More like the diameter of the shaft. The fastener will go in that hole. It functions like thumbtack or harpoon, with the fin edges bending back on insertion and catching the side of a piece of interior plastic or metal, and holding one or two pieces of plastic shrouding to the door or other surface.
If you have never cleaned that area before, if may just be left over from the assembly process. We used to keep spares of those at our shop because we would occasionally dropped them down into some dark place when putting panels back after working on a door, heater, glovebox, or anything else that had a plastic shroud over it. Rather than spend all day fishing it out, you just put in a replacement fastener.