Okay. I took a quick look around and came up with 4 options.
1. Seat belt bolt - It's really not that big of a bolt. If the lug you have on the end of the ground wire is a circle, you could cut some out of it to make it more like a "U" and slide it under the bolt.
2. Cargo tie downs - The tie downs have a metal plate on the bottom. You could run the ground between the bolt head and the tie down plate.
3. If you have a soft top, there are holes on the rail where the hard top bolts down. Just pick up the proper size bolt and use one of those.
4. If you remove a tail light you will see there is a tub substructure there. By removing the corresponding trim on the inside, you would be able to drill whatever size hole you prefer and run a bolt through there. With the tail light removed, you would have access to install a lock washer and nut on the bolt.
That's after a quick glance. I'm sure there are many more options. No matter which you choose, make sure to remove any paint so you get good metal-to-metal contact.