Okay, here is the question. I have found mine on the floor too. Does it go on from the top or the bottom of the mirror support?

 

look up under the mirror and see where the arm meets the windshield, there should be a lip on the arm and a bracket on the windshield. The cover in question slips over the two of them.

If you are looking directly into the rearview mirror, the cover should slip on pushing toward you with the open side of the U-shape facing you and obviously the finshed side of the cover facing down.