This is by far the easiest mission-based trophy and the easiest way to get an S-rank on a mission.
Stay in the vehicle the entire trip until it comes to a stop, wait for the driver to leave, then sneak behind the building on the left side. You'll come to a ladder, climb up and head to the tower in front of you. There is a security camera right inside the doorway, so peek around and take it out; then grab the cassette tape on the desk. Sprint back to the truck as a soldier will make their way towards the tower to check the camera, but you should be long gone before you are spotted.
Take the same route back to the truck and lay down inside. You'll have to wait a few minutes (depending on how quick you got the tape and came back), but the driver will return and drive out of the area. Just be sure to stay prone so you aren't spotted while leaving.