After opening the roof door (or avoiding the door by taking a running jump across the street instead), as shown in the previous section, take a running jump to grab the horizontal crevice on the next building to the north. (Aim for the right end, where there's no overhang.)
Traverse to the left until you can pull up.
Go around the corner to the right and climb onto a block ledge near a small, rectangular opening through which you can peer out onto the rooftops.
If you didn't kill that sniper earlier from below, he may start shooting at Lara through the opening, and you'll need to take care of him.
Look through the opening and locate the electrical box across the street.
Use the crossbow fitted with the laser sight to aim at the red light above the box. One normal arrow should be enough.
- OR -
You could also use the pistols. Lara won't auto-aim, so you'll have to stand near the opening and tweak Lara's angle bit by bit until you're shooting in the right direction.
The box doesn't explode, and the light doesn't go out, so it's hard to tell if you've actually done anything. But when the box starts to smoke, you'll know you've disabled it.
Doing so opens the big door below and to the left of the electrical box.
Return to the ledge where you pulled up, and safety drop to the street in the starting area.