Red circle wire and connector

Anyone tell me what this wire is for? I believe it’s either a dark brown or black, no stripe. My quick searches of brake wiring shows brown as running lights possibly? That part makes sense with no power to it until you pull the headlight switch. It only gets 7v to it so I’m guessing it’s a small bulb like a 168/191/etc bulb. If my findings are correct, then more importantly, what do people have it connecting to?! I have all lights including tag lights.

Thanks in advance!

 

LICENCE PLATE LIGHT WIRE..
back in the old days, trucks were usually delivered to dealer with no rear bumper, and the license plate mount and light power wire were mounted there on the rear most crossmember.

 

Thank you! Seeing as I have those already, I’ll cap that off to avoid any future issue. I just wanted to make sure nothing would be missing.