if u want under the seats, under the drivers seat is prob best. I would also recommend a simple mounting to one of the rear folding seats as well, both places work great. Some people have even installed their amps in the storage compartment in the rear.
Seat bolts are not a great ground IMO. I would suggest sanding the paint off of an area close to the seat bolt hole, make sure there isnt anything under it, and attach your ground ring terminal to that bare metal with a self-tap screw. I usually suggest putting a screw on the side of the terminal that prevents it from loosening itself. After, if you can apply some spraypaint to the underside where the screw pokes through, it will prevent rust.