Replace battery under rear seat cushion

The interior fuse box in a Pontiac Bonneville is underneath the backseat on the driver's side.

To remove the rear seat cushion:
1. Pull up on the front of the cushion to release the
front hooks.
2. Pull the cushion up and out toward the front of
the vehicle.

To reinstall the rear seat cushion:

1. Buckle the center passenger position safety belt, then
route the safety belts through the proper slots in the
seat cushion. Don't let the safety belts get twisted.
2. Slide the rear of the cushion up and under the
seatback so the rear - locating guides hook into the
wire loops on the back frame.
3. With the seat cushion lowered, push rearward and
then press down on the seat cushion until the spring
locks on both ends engage.
4. Check to make sure the safety belts are properly
routed and that no portion of any safety belt is
trapped under the seat. Also make sure the seat
cushion is secured.

Notice:
The battery and main fuse blocks are located
under the rear seat cushion. The battery's ground
terminal and some relay wires are exposed.
To help avoid damage to the battery and wires,
be careful when removing or reinstalling the seat
cushion. Do not remove covers from covered
parts. Do not store anything under the seat, as
objects could touch exposed wires and cause
a short.

The battery is located under the rear seat cushion. To
access the battery, see "Removing the Rear Seat
Cushion" in the Index. You don't need to access the
battery to jump start your vehicle. See "Jump Starting"
in the Index.

it is under the back seat. you need to lift out the bottom part of the back seat to get to the battery.

It will be under the back seat.

To remove the back seat pull up on the front of the cushion. The front of the cushion will pop up. Pull forward and out on the cushion.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

The battery is on the passenger side under the lower seat cushion. When removing the rear cushion make sure you disconnect the wiring for the rear heated seat cushion if it is equiped with heated seats

There are 2 fuse boxes, they are called Bussed Electrical Centers.
The front Bussed Electrical
Center (This is one fuse box) is located in
the front of the engine
compartment on the
passenger’s side. Lift the
cover to gain access

Rear Underseat Bussed Electrical Center (BEC)
The rear BEC is located
beneath the rear seat on the
driver’s side. The rear seat
cushion must be removed to
access the BEC. See
“Removing the Rear Seat
Cushion” in the Index.

Download a manual at this site:-
http://safemanuals.com/user-guide-instructions-owner-manual/CADILLAC/SEVILLE%202000-_E

"The battery is located under the rear seat cushion.
To access the battery, see “Removing the Rear Seat
Cushion” in the Index. You don’t need to access the
battery to jump start your vehicle. See “Jump Starting”
in the Index."

Its on passenger side under rear seat cushion. Get in back seat FACING rear seatback, grab under lip of bottom seat cushion, and pull straight up. Disconnect seat heater connector (if so equipped) on passenger side and remove seat bottom. Tada! There ya go! Oh... btw. Its a $150 battery due to the venting system designed into it to vent fumes through a tube to the outside world. Thanks Cadillac!!! :(

please rate highly if this is what you need

Section 01-10: Seats and Tracks 1994 Mustang. Workshop Manual
REMOVAL AND INSTALLATION Seat Cushion, Rear Removal and Installation

1. Release pin from locks by pressing release button under rear seat cushion.

1. Pull rear safety belt and buckle (60044) through openings in rear seat cushion pad and frame (600A88) and remove rear seat cushion pad and frame .

1. To install, reverse Removal procedure.
   

----------------------------------------------
Section 01-10: Seats and Tracks 1994 Mustang. Workshop Manual
REMOVAL AND INSTALLATION Seat Back, Rear Sedan
Removal

1. Remove rear seat cushion as outlined.

1. Remove rear seat bolster. Refer to Section 01-05 .

1. Remove rear seat bolster mounting bolts. Remove bolster assembly.
2. Remove four seat back to package tray retaining bolts.

1. Remove seat back.

1. To install, reverse Removal procedure.

Not 100% sure on yours but normally you'll either have a couple of bolts way down under seat cushion at front, near floor, or in some cases, you have to push seat cushion back toward rear window, hard & lift, to pop it out of notched catch under it.