He is found in the Junkyard Level. You will see a plane flying in the background, you need to shoot this down with a missile. The easiest way to do this is get onto the roof of the Incinerator (the round building with ricochets on top and flames on the sides). As the plane flies by to the left (the left if you are facing the elevator/crushing thing) shoot homing misled a little in front of the plane to knock it down. Once the plane falls go down into the pit near the environment trigger (the gun on the wreckage) the plane will have crashed into he side of a building down there and a hole will open up. drive down it and collect the weapons. At the bottom there is a control panel, shoot it and it will lower Yellow Jacket.