can't find ground wire for '09 subaru wrx

This is my first install and I can't find the ground wire. I have three harnesses. One is the main one without a black wire, the second is a smaller white harness with only 5 or so wires which includes a thicker cable wrapped in black electrical tape. The third is a single black and blue cable with grey spots on it and it's end says AMP.

What do you think?

Thanks,

Heath

Run a self tapping screw through the ring terminal and into the bar.   Then run a second screw through the bar, placing the screw directly on side of the first screw.  Have it close enough so the head of the second screw keeps the first screw from backing out of the hole.

Perfect. Thanks for the explanation.

I am praying to have this finished up tomorrow. I feel like such a redneck with no head unit and wires strewn around the dashboard.

Thanks,

Heath

Turns out that single black wire is a ground, not sure if that's what it was intended for, but I was told by a technician that it would would work fine. For a lower ground for the satelite radio I inserted the cable under the bolt that affixes the seat bas to the floor.

Thanks for the help!