The grounding strap coming off from the battery is bolted through the floor pan under the back seat. It appears that will be one of the weak spots in the 2000+ series cars for the electrical system. It is bolted to a painted surface, and neither the lug or the bolt are very large. The bolt extends out the bottom of the car, so rust starts to build up. It would be a good idea to remove that bolt, and clean it and the threads in the floor pan as good as you can. I also used a star washer under the lug to help bite through the paint for a better connection.
Also check out the demo vehicle at http://www.alldatadiy.com/
One of them is a 2003 Bonneville
that should have all the information on grounding points and other useful information.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.