1987-8 Cadillac Deville rear reflector.

There are two of these on each side on either side of the license plate.  

GM #1650874  There is a number 3 or 4 following this...however, they seem the same in all respects.  17 3/16" X 1/2"  There are three mounts and each has two nuts with it...I hope you saved your old one otherwise there is always glue!  The pictures shown below as examples are not correct.  These reflectors are not mounted on the bumpers.