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I would guess it to be a Yagi cell phone antenna, or something similar. A picture that is taken closer to show more details would help but I am thinking it is some kind of antenna.
The van that delivers my wife's oxygen supplies has something similar to the black thing mounted in the same place. It's a light that illuminates the area behind the van for ease of loading and unloading at night.
In studying the picture, it may well be a light but a closer picture would help. Am I correct in assuming that the picture is of a van that isn't close by where you could get more pictures? Maybe in a for sale advertisement?