After the arena, look at the car parts and click on all the hatches to reveal all the numbers. There are five in total. Exit the area to head back to Corley Motors. If you move your cursor to the left of the factory, you will discover a right pointed arrow. You will arrive behind the factory. On the left, you will see some meters. The easiest way to find the right section of wall is to use the right stick to "snap" to specific locations. One of them is on the wall near the ground. It is below the first pole to the right of the meters. When all the meters lock and display a black line, use your foot on the wall revealing the secret door and your achievement.