I had one of those fall out of my 91 tail light, iirc it was a reflector for the turn signal or the brake light. I'll go out and take a pic if I can get to it.
EDIT: OK I am full of fail today, it is a reflector for the reverse light, and it mounts to the top of the lens housing in my 91.
This picture was taken with the lens installed in the car, looking up towards the top.