The battery is right about in the middle under the back seat. It is very easy to pull the seat out. Just push back a bit, and firmly lift up on the front. It will come right out. The only issue with the battery being back there has been the bolt for the grounding strap to the body getting corroded or rusty.
If you are thinking of replacing it, be certain to get the proper one for the car. It has specific venting requirements due to its location. Once you pull the seat, you'll see what those are.
It'* on the passenger side. There is one clip on each end of the seat .You grab a hold of the bottom of the seat where it meets the carpet and pull strait up. Yeah, the battery stays real clean under there.
The seat on my 03 just lifts straight up. It doesn't seem to have a slight hook catch to keep it down.
The battery is special size 100 for the vents that have to have the rubber tube system to send the vapors out under the car. You can check the green/yellow/white eye on the battery just like on all Delco batteries for it'* hydrometer status.
I keep asking Walmart when they're going to start stocking that type battery. I like their last MAXX battery I bought 6 years ago for the other Buick.