It’s not surprising that you’re having trouble finding the Prius 12V battery location—it’s hidden pretty well! You can find it in the rear hatchback area.
To get to the battery, you’ll need to remove the floor mat and the carpet. Underneath, you’ll find a plastic cover that is covering a hidden compartment. Remove the cover. Inside the compartment, you’ll see a plastic storage bin. Remove that as well.
With the bin gone, you’ll see an even deeper compartment where the spare tire is stored. To the far right of the spare tire, there’s a piece of plastic molding—it looks like a piece of the compartment wall, but it can be removed. Finally, when the molding has been removed, you will see the battery.