If your battery is stowed somewhere hidden, it can be hard to find—and in the 2007 Lucerne, this is especially the case! Contrary to typical battery placement, the 2007 Buick Lucerne battery location is beneath the rear seats.
To get to it, you’ll need to push down on the rear seats lightly, then lift up and remove the seats entirely from the vehicle (you may need to ask for assistance for this part, since the seats can be surprisingly heavy!). Underneath, you should see the battery sitting right under where the middle seat is.