Shortly after reaching the next area, pull up the map and analyze the movements of the guards before jumping out. Jump into the truck and crouch in the back out of the sight of any guards who may detect you. Make your way around the crates and pick up the gear. It's best to take him out as well, instead of trying to run around in a stealth-like fashion. Take his clothes and drag him back to the manhole area, out of sight of the patrolling guard located in the north region.
Wait until the guard's back is facing you, then use whatever means of putting him into an unconscious state (I chose the silencer pistol, again). Climb up the ladder, but be sure to check that the guards aren't patrolling directly over the manhole.
Open up the map and located the red arrow and proceed across the sewer walkway until you reach the opening which leads to the surface.
