Look under the back seat. Push in on the front of the bottom seat then lift up.
it must be located somewhere at the rear end. check it thoroughly. http://www.buick-lucerne.com/2010-photos-collection/
its under the rear seat. push the rear seat bottom towards back and up to remove and u will see the battery in the car.
It's under the back seat saddle, just pull up on front edge.
Under the back seat just like an oldsmobile aurora.......
Search Buick Lucerne Questions
Buick Lucerne Experts