After venturing down the hole in the building, you should get to an old subway after you explore the sewers. After killing all of the men down there, do not go through the door. Keep going strait through and up a catwalk. Before people in the subway notice you there, tell your men to hold their fire. At the bottom of the catwalk on the other side, you will see a yellow generator. Shoot it, and it should kill the lights. Put on your night vision goggles and be careful since they will still shoot at you but will usually miss. Tell your men to open fire and move through the subway to your next checkpoint.