The 2004 Cadillac DeVille battery location is beneath the rear seat.
To access the battery compartment, start by releasing the pressure clip on both sides of the rear seat. Once you have moved the seat out of the way, you’ll see the battery.
Use a wrench to release the brackets. Then, disconnect the vent hose and terminals before removing the old battery. Remember that the negative terminal should come off first to prevent blowing a fuse!