Mystery wire solved. It's the original ground wire for the tank. I guess it wasn't reused when somebody r&r'd the sending unit before I bought the truck.

By the way- Thanks to CaptKaos for the writeup in the Tech section detailing how-to on the tank/pump replacement.it was really handy.
But I'm a little disappointed. When I opened my box from NAPA there was only the pump and seal pkg-no video camera in it...