I have a black ground wire that is bolted with a 10mm (I think) bolt near the underside of the driver side transmission mount.

Does anyone know what this wire gets connected to?

 

It's just another ground, can never have too many. Bolt it from the trans to car somewhere, firewall, frame rail (actually the unibody), whatever. I don't remember what it's bolted it in it's stock location (It's gone in my car and I've got a big fat 4 gauge wire going from the trans to the unibody instead) but it doesn't really matter, as long as it's steel and it's part of the car's body, it'll work.