The Barn Finds are hidden garages containing wrecked legendary cars that you can restore in your garage and then add to your collection. There are 14 barns in total in Mexico.
You can't discover a barn on your own before it has been "activated". Barn Finds are "activated" from the "barn rumors" as you progress through the game. A hint circle, which marks the general area where the hidden garage is located, should then appear on your map. You can then go to the marked area and find the barn.
Restoring cars takes a while. Once the car has been restored visit your garage, go to the "Barn Finds" tab, select the appropriate car and select "Collect".
Below you will find pictures of all the barns which should help you locate them and pictures of the cars after they have been restored.
