I don't know that I would consider under the back seat as being in the trunk. The only thing sealing the area the battery is in on current production Buicks and Cadillacs is the foam of the seat cushions. The battery itself is mounted in a recess in the floorpan, about 2" behind the footwell in the rear passenger area. The venting of the battery is the key, though. At one time (I beleive this has changed), the company I work for was the only aftermarket source for the group 79 vented sidepost battery for those cars, otherwise it's a dealer item.
I agree 100% about learning the basics and the whyfors and howtos before jmping in and altering things.
