2012 Mercury Mariner, see the pictures. What is it and where does it go!
That’s an electrical connector. Maybe for a tail light, or the electrical power connector for the tailgate. Do all of your lights still work? running light, brake lights, backup lights. If one of them is out, there’s your problem. How about the wiper on the back glass, or the washer nozzle for the back glass. Does the electric lock for the tail gate still work. If one of the above is no longer working, you’ve probably found where it goes.
That looks like it’s on the left side of the vehicle, so focus on things on that side first.
If none of the above, maybe a connector for the brake ABS sensor?
Did you buy this secondhand just recently? Did it have a trailer hitch?
Auto manufacturers will sometimes install a wiring harness where not all the plugs are used, and the wiring diagram will state, “NOT USED”.
Feel the four white stubs sticking out of plug. Are these plastic rods instead of wires? If so, then that’s what you have.
If nothing is malfunctioning . . it’s just the trailer-ready plug that had been tucked up in there.
It looks like a trailer-light connector.