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. 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. You should see a fallen sign with red letters to your left. Climb up the debris to find it. If you crossed two electric posts you've gone too far (again these posts are to your left).