Don't follow the main road to the base. Instead, take a right after the first bridge. Follow the canyon to the bridge you have to blow up then go under it. You will encounter a BMP. Take out your LAW 88 anti-tank rocket launcher before you go under the bridge. After that, walk up the canyon until you find a slope to your right. Walk up the slope and you will find yourself back on the main road. Follow it until you find the building where Foley is held prisoner. Release him then do whatever s needed to complete the mission.