I found where the convoys spawn. You must go to Diamont Basin northwest at the main road and find the point the road curves. Climp up the rocks, move, kill or hunt something near the road and a convoy should appear. I wanted two convoys so after killing the first one (with bare hands to get the trophy) I went far away to another region and returned again at the same point (so the previously killed convoy would clear and load the region map again). After a minute or so of foolling around the second convoy spawned. Click the link to see where is the point on the map. The road is marked with red.