In the location in the image below there is about 20 pinata truck almost lined up in a straight line for you to get, once you get the first one just follow the trail and there will be a circle of trucks and then it will lead out to the other side of the dunes.

After every race you do it resets all pinata trucks on the map so all you need to do is go back to this location on the map, it should only take about 10 or so trips until you get the 150 achievement.