For future reference in case this turns up in searches, the FAQ sticky
has pictures & details on this topic.
Funny how the more things change, the more they stay the same. My '64 Corvair had a drop-in holder/pocket for the battery in the rear of the car (motor was there also
), and it is under the rear seat in the 99+ Bonneville and my '94 BMW.
Gotta love those Beemers, alot of them had there batterys in the back, and why? im not sure, I mean in the Acadia theres no room really under the hood, but most beemers are ok,