Head 2 blocks east, take a right, go over the bridge. When you see a guy with a sombrero on, ask him his name. If it sounds Mexican, proceed to the nearest bathroom. Flush the toilet, open the lid and a key will be there. When you step out of the bathroom you will be looking at a small hill with 3 big cactuses on it. The second cactus is the direction you will head. As you cross over the hill make sure you keep your heading. The barren looking building on the left is the one you're looking for. Go around the back on the right side. The second door in is the one. Knock three times with a 2 second pause after the first, and a 1 second after the second. When the door opens ask for La Migra. Take the vial out of his left hand, and leave 200 pesos next to the book on the table by the door.