There are still secondary objectives. Follow the road to the southern part of the town and enter the marked building, where there is a map of the truck's route (M7,12). In the building, there is only one guard on the 1st floor. All you have to do is place any trap on the truck's route (even a trip mine) to eliminate the convoy. In fact, the only requirement is to stop the head truck and you do not need much to do that.
To leave the building quickly, with the plans on you. Walk north on the top floor, exit the building and dash to the 1st floor of the restaurant. Then, jump down on the northern side. There, on the turn, you will encounter no opponents.
Finally, approach the last cannon (M7,13) and climb up the stairs. There are only three opponents there. This time around, you can plant explosives after you distract them (or use TNT).