These wires are in the rear passenger side, beside (underneath) the fuel filler. They look like 2 black wires going to 1 female spade terminal. Are they ground wires? Any help? Thank you.

There are no stock wires in that area. The wire harness typically goes up overtop of the evap tank, goes through one of the holes in a support beam at the back, and then runs along the back panel beneath the tail lights. There is a branch that goes down through the hatch floor to the fuel level sender, and fuel pump. This branch has a ground lug that attaches under the hatch floor above the gas tank.

What I see is a harness in the bottom of that area below the filler that normally shouldn't be there.

That's where I put the harness. Now I know where the wires go. Those wires still come out of harness.

ah, now I understand. I am totally baffled as to the use of those wires. There are no electical components in that area that these could be connected to.

Check if this connection is connected to ground with a multi-meter, or if it gets +12volts when the key is on or lights are one, or ????

Possible it was wires for speakers added by a past owner? Taped onto the existing harness? Other Guesses?

Wait! I see two wire harnesses in that picture. Me thinks you have the harness that is supposed to split off and run along the roof to the interior light laying in the back there. That connection is the one that goes to the spade connector on that roof light. No fun to re-route that harness....

Another possible clue: to the best of my recall, all the factory connectors in that area are bullet style, no spade style.

The rear window defrost power connectors are female spade type, for the 76 280Z at least.

Spade type on an early 260Z also. The female spades on my 260 are 90` and are a sgl. wire to each one. Both located on the top left and right of the rear window. The one pictured doesn't look like it's long enough at least for a 260. 

 I'd go with zKars advice and test it for ground continuity first & then for voltage with the key in all positions. if you find it has voltage, pull fuses until you kill it. Then you'll know which circuit it's on.

 Just remembered, Zs wiring does use a color code. ie. lighting circuit uses red base with colored stripes. Pull some of the tape back. You may get a clue from the wire color.