An icon, blue truck, will appear periodically on your mini map. This will lead you to a truck which you can either destroy or liberate. To get the trophy you need to liberate it.
The truck has one single enemy and there are two ways to get this trophy. Either pull up beside the truck or ahead and shoot the driver, or you could unlock the skill which allows you to takeover a vehicle by jumping from your vehicle (land or air) and into the driver or passenger side, killing them. The latter is obviously easier.
You will then be guided to the nearest appropriate outpost which you have liberated.