Answerthe battery for the 2000 deville is usually behind the back seat*EDIT* To be more clear it is UNDERNEATH the back seat not behind it. Lift the portion that you actually sit on and you will find it.
Push very hard at the center of each back seat (left and right seat) towards the back of the car. Then, while pushing back, lift up. It will pop out of its catch.
Under the rear seat
Under the rear seat. Lift on the front of the rear seat and should pop out somewhat easily.
under rear seat cushion there are fuses you can remove pass side push on seat lift up cushion
Remove rear seat and remove battery
you push in the 2 small black levers and lift - they are located on each side of the back seat at the bottom
Located under the rear seat, two release handels underneath rear seat.
In the front right of the engine, and under the rear seat.
Lift the third row seat cushion and the jack is supposed to be in the tool bag under the seat
Look under your back seat. I have a 2001 Seville and that is where mine is located. The seat pops out very easily.
Move the front seat forward and remove the bolt covers on rear of seat bottom, Remove the 2 torx head bolts, unplug connector, and push bottom upward and lift to unhook front track and lift out of car