Cargo Truck quests are one of two random side missions, in which you will have to either destroy or hijack the cargo truck. You will not know when a cargo truck has spawned, unless it’s close enough to you on the map, where you will see a truck icon. The cargo truck will not really stop, unless it gets stuck, the driver is turning around or you kill the driver.
There’s two ways to complete these quests, one of which is by destroying the truck, giving you some Rupees and XP. The other way is by hijacking the truck and bringing it to one of your liberated outposts. If you do this, then you’ll get some extra Rupees and XP, as well as having all your currently equipped weapons supplied with full ammo.
In order to complete the game 100%, you will have to bring three cargo trucks to outposts of yours. Bringing two will reward you with the Warrior signature weapon in the shops, while three will net you the Robin Hood achievement/trophy .