This part can be found immediately below the main bridge near the mission starting point. To reach this area, jump down to one of the side bridges and walk to where it ends. Turn to the opening below the main bridge, and land on the nearby scaffold. There should be a large gap existing between this scaffold and the area below the starting point. Begin flying in that direction and land on the highway. You will find the part at the edge of this highway. After obtaining the part, complete the mission to unlock it.
