There are 15 Barn Finds to collect in Horizon 3. These are found scattered across the map in various areas and show up after a certain amount of play time as your mechanic friend will let you know of some car rusting away in a barn. It'll show an indicator on the map of where the barn find is located. Head over to this area and then search for a barn and drive into it. Once that's done it'll trigger a cutscene, the car will be taken away and restored.
The achievement unlocks once you've restored fifteen cars. I've included a full map with 15 barn finds below along with a video showing the location of each barn find. The map included is 4k in size and courtesy of
Gamerheadquarters , click on it for a full view which should make it easy to spot barn finds.
Map
15 Barn Finds Cars:
-Dodge Charger Daytona Hemi (1969)
-Ferrari Dino 246 GT (1969)
-Ferrari 166mm Barchetta (1948)
-Ford XB Falcon GT (1973)
-Ford Super Delux Station Wagon (1946)
-Holden Sandman HQ Panel Van (1974)
-Holden HQ Monaro GTS 350 (1973)
-Holden 50-2106 FX UTE (1951)
-Jaguar MK II 3.8 (1959)
-Lamborghini LM 002 (1986)
-Maserati A6GCS/53 Pininfarina Berlinetta (1953)
-Meyers Manx (1971)
-Nissan Skyline GT-R V-Spec (1993)
-Reliant Supervan III (1972)
-Toyota FJ40 (1979)
Edit: Note, it would appear some people are having the achievement get stuck at 93% despite restoring all vehicles. You're not the only one, with the
newly released Barn Find you can apparently get the achievement by collecting/restoring it since it's fixed quickly. There are now more than 15 cars available, you only need to find 15 of them