Head to Churchtown and meet up with the Rakyat leader to get the C4. Plant the C4 on the truck then quickly climb the water tower. There's a sniper rifle waiting for you there and some supplies as well. Wait for the convoy to arrive then provide covering fire to your allies below.
Once the enemies are eliminated, climb down the water tower and ride the cargo truck. Catch up with the convoy and eliminate the remaining enemies then open the APC. Watch the next scene.