I talked to Andy through PMs, but I'll make my 2 cents worth public to add to the thread.It was either added at the dealer or by a PO, and I have two scenarios in mind as to why.
1. The owner simply wanted a decorative tag on the back, and lighted so it could be visible at night.
2. It was a plow truck or some type of utility use vehicle and a tag was added with something along the lines of "Plow Vehicle" or "Caution: Slow Moving Vehicle" on it and needed the light for the same reason. I'm leaning toward this one considering how rusty it is.