Rescue the escaped prisoner
You can try to rescue the prisoner after you have reached the upper part of the Wakh Sind Barracks, still before you have found the engineer, or after you have fultoned him. Examine the ledges near the top viewpoint (it may be occupied by a single guard). The prisoner has been shown in the above screenshots. After you reach him, you only need to fulton him away, which completes his secondary objective.
Stealing the 4x4 vehicle
The car that you are looking for is the off-road and it travels between the barracks and the outposts in this area. At the beginning of the mission, the vehicle should appear near the path to the main barracks entrance (use binoculars to identify it easily), but if you miss the moment, you can wait for it to reappear here, or check out a different point on the map. You need to sneak up to the vehicle and fulton it, i.e. send it to the Mother Base using the balloon.
Note - It is impossible to fulton the vehicle when you are playing this mission for the first time. For this, you need the CARGO 2 upgrade that allows you to fulton containers and other heavy equipment. The earliest you can unlock that is after the tenth main mission, because one of the requirements is the transportation specialist on the crew, and you find him in this mission. After you obtain CARGO 2, replay this mission and fulton the vehicle.
Find the blueprint in the barracks
What you need to find is blueprint [UN-ARC] and it is in the upper part of the Wakh Sind Barracks. You need to check out the four small buildings close the where the engineer used to be kept. Watch out, because the area is being patrolled by several groups of enemies and there can also be single ones in the buildings. You need to enter the building shown in the above screenshots, because this is where the blueprint is.