1999 Ford Windstar reverse/brake/passenger side headlight a

Car manufacturers sometimes install wire harnesses which fit a variety of similar model trim types (For example Windstar, Windstar GL, Windstar EX and so forth) in order to reduce inventory and cost. In other words, a Windstar model with all factory accessories installed with all bells and whistles can quite possibly have the same wire harness as a base model. Therefore, the base model will have wire connectors which seem to plug into something, but actually don't because the base model does not have the factory installed accessory.

First, check to see if any of your accessoris are non-functional.
As long as the wiring has not been tampered with too much, the male and female terminals (or plug and socket) should be within viscinity of each other. A good indication of whether the terminal you have in question actually plugs into anything is its physical appearance. If the terminals inside look dirty, dusty, or if there are no scratch marks on the plastic casing of the terminal you mention, it may well be just one of those extra terminals the manufacturer included in the wire harness to satisfy an accessory on another model Windstar.
If this is the case, not to worry. Just leave it alone. Don't cut it off; that can do more harm than good.

Finally, if you can identify the wiring colors, you can consult a wiring diagram to obtain possible answers to "Where does this thing hook up into" because every accessory generally utilizes it's own wiring color code.
Hope this helps.