The game will hint you that to find the truck, you will have to obtain intel data first, which tells you about the convoy's route, after which you should locate it and start following. However, these are secondary objectives, which you can ignore and still complete the mission (they have been described on one of the following pages of this walkthrough). Instead, you can head immediately towards where the truck is parked.
Your destination is Nova Braga Airport. It is best to approach it from the North or from the West. It is necessary that you dismount the horse after you arrive in the area, because there is sniper occupying the flight control tower, who may spot you from a very long distance. Try to reach, as quickly as possible, the walls surrounding the airport, thanks to which you can hide away from the sniper.
To get into the airport, check its walls to the West. You an open the gate or climb onto the crate and over the wall. There is a downside to the latter, which is that you cannot then escape the airport along the same route, if it is necessary.
The truck is parked in the North-Western corner of the airport - the exact location has been provided in the above screenshot. Before you come close to the truck, decide in what way you want to complete the mission (see details on the next page).
Note - If, in the marked spot, you didn't find the truck, this means that you are late and the truck, most probably, joined the convoy of two APCs and is heading along the main road to Kiziba Camp, north of here. In such a situation, you can catch up with the convoy, or head for the camp immediately and wait for the truck to arrive in its destination.