Since you neglected to add what year Frontier you have, makes it more difficult.
I have 2004 and 1998 Frontier, and I used a Hopkins #43515 kit which just plugged into my existing wiring connector under the rear of the bed, on passenger side. This kit not only included the 4-pin tow connector, but also had the converter for vehicles like Nissan which have separate brake and turn signal lights. So I would think a big outfit like U-Haul might have used a similar plug-in harness, and all you might need to do is disconnect that, and plug the stock connectors back together.
I have 2004 and 1998 Frontier, and I used a Hopkins #43515 kit which just plugged into my existing wiring connector under the rear of the bed, on passenger side. This kit not only included the 4-pin tow connector, but also had the converter for vehicles like Nissan which have separate brake and turn signal lights. So I would think a big outfit like U-Haul might have used a similar plug-in harness, and all you might need to do is disconnect that, and plug the stock connectors back together.
