Two lug nuts on the rear seat in the trunk
The battery is located under the rear seat.
Use an axe
Just pull the seat up to remove safety latch and it will come out
On a 79/80 Deville there are nuts under the seat that have to be loosened and be careful of the power seat wires and mechanisms under the Seat.
Cadillac did not produce a DEVILLE convertible in 1983. ELDORAO yes. If it is a 1983 DEVILLE convertible, its custom made. To start remove the lower rear seat cushion and inside trim. Sorry I cant help further not knowing its customizing technique.
pull up on the front part of the seat
I believe its under the rear seat.
In the front right of the engine, and under the rear seat.
The battery is located under the rear seat (passenger side).
Behind the rear seat, access through trunk.
It is located under the rear seat. The seat is easily removed by lifting sharply on the lower front lip of the seat.