If I remember right, you have to trigger an event off to the right side of the area with lots of cells (room called stage 3?) where the Ura? (the fist fighter) gets tricked into chasing a guy with glasses into a hidden room at the back of stage 3. Then you have to go back to the room with 3 switches, toggle the one on the left, head back to stage 3, go through the now unlocked hidden door and rescue Ura. The guy with glasses should then cough up the missing switch, which you can then toggle and then go through the rear entrance of stage 2 (the one to the left in the large area that has stage 3 on the right).
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