I have an '04 Lariat Screw. On the right side of the license plate frame on the rear bumper is a small round plastic cap. I thought that it was a 12V power plug, but when I flipped open the cap, it was a lock. When unlocked, the lock itself pulls out of the bumper, leaving a small hole where the lock was.
What is this for? It does not unlock the spare tire and I can't find anything explaining it in the manual.

 

I noticed this too when inspecting them at the dealer. I really think it is for the spare tire, though. Once you unlock it, you stick the rod in that hole to lower the spare, i'm guessing. Hope this helps.

 

I just looked up spare tire in the manual and you are correct.