The rear seat bolt was the natural choice for the amp's ground wire attachment point, but the hole in the ring connector that came pre-attached to my ground wire was too small for it. Others have put new holes in their trunk floor to ground the amp, but I didn't want to do that.
I found some nice bolts next to the left rear seat latch. It involved taking a panel or two off to get to them, but it was worth it I guess.
This is the panel that had to come off to get to them. There were two Torx 20's to remove before the rest of it just popped off. Oh, and there's a wire that connects to the trunk light there so remove that before you take it totally off, as is pictured. You can see I loosened up the beige pillar near the door frame, too:
Here are all of the lovely bolts to choose from, right under the seat belt tensioner:
And here's the ground connection I made. I sanded it like hell and confirmed an adequate ground connection via multimeter:
The wire runs under the side cloth and pops out right next to the amp; very clean looking.