Near the Cave of the Abaddon, behind the warhead south-east of the Cave entrance. Once you've detonated the warhead, it's on the ground to the north east of the pipe's top opening close to the mountain wall. If you've already destroyed the warhead and missed the ED-E upgrade fast travel once again to the Cave of Abaddon and turn back outside. Follow the mountain wall on your left. It's N of the twisted red letters (WORLDSHRP) on the fallen building. If you crossed two electric posts you've gone too far (again these posts are to your left).