At the Junkyard level, shoot down the airplane that circles around the level. The easiest way to shoot down the plane is with a homing or fire missle while no enemy is around you. The plane will catch on fire, circle, and crash into a wall in the lowest area of the level, that leads to an underground tunnel. The suspended crate's opening faces this area. Follow the tunnel under the sewers, collect the weapons and health, and continue until you see a control panel attached to a large column in the middle of a circular room. Shoot the control panel and a message stating Yellow Jacket has been unlocked will appear. Yellow Jacket will appear inside the destroyed column.