That device is called a tail light monitor. The cover is, appropriately, called a tail light monitor cover.
It has no wires, only fiber optics, when the tail light lights, it sends the light through the fiber optics so the end of it glows, and you can see if it's working in your rear view mirror.
Replacement covers made out of metal are available at many Airstream dealers, just do a quick search.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.