It goes on the c pillar lol.
The a pillar is the pillar that are on either side of the windshield.
the b pillar is the one right behind the drivers door.
the c pillar is the one right behind the back seat and it goes where the stock seat belts are. Im pretty sure you have to remove the seat belts but its not like anyone will be able to sit back there anyways (with or without the bar but especially once the bar is installed).