Found THIS on a quick web search. It looks like it sticks to the inside of the rear window. It's LED, which probably requires a 12V system.
You want to put it as high as you can so it's visible to the tailgaters. Down under the tailgate is not a good spot, visibility wise.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.