You will have to complete a few races before you are told about a barn find that can be ANYWHERE on the map. This barn is located in the top right corner of the map just south of the white road. You can view the exact location in the video below. You will be able to spot a small rooftop using your map, and that's where it is located.
After driving to this location, Ashley will pick it up and let you know when it's ready. After a few races, head back to your garage, to the barn finds tab, and pick up your new
MG Metro 6R4!
After picking it up, the achievement will unlock.
The car is in the Group B Monsters class, and you'll also want to win a race with it in order to unlock the following achievement: