Everything that I have read says that it is located behind the passenger seat underneath a panel. It doesn't say exactly where, but it's back there somewhere. Pull up every carpet and panel and plastic covering you can. It's back there.

I know lightgear is right and we have pulled up metal strip and carpet and can't get anything else loose to access the battery...Find it hard to believe that Cadillac would put a battery in an inacessible place.  Owners's manaual sure is NO help!  Will keep trying...Marcia

The battery is right behind the passenger side seat, you will find a keyhole in the side panel, use the ignition key to open it and pull the whole panel out. The panel is carpeted and it is on the side NOT on the floor.