The wire fits behind the nut and washer on the inside of the Z-plate. The two front mounts for the foot rest are studs; the rear one is a bolt that goes through the rest and Z-plate to anchor the ground wire, too. If you have the parts book, look at the blow-apart diagram for Group 15, and it should clear things up.
I ran my bolt through the other way so I could replace the plain nuts with 3 polished stainless acorn nuts, backed by shakeproof washers. The ground wire is behind the head of the bolt on the inside of the Z-plate