https://www.gamespot.com/articles/forza-horizon-4-barn-find-locations-and-vehicles-w/1100-6462084/

Forza Horizon is back for another year, and that means Barn Finds are back too. If you're new to series, all you need to know is that as you play through Forza Horizon 4, you'll periodically get a call telling you that there's a beat-up old car somewhere for you to find on the map. This will unlock a particular Barn Find, and you'll be given a rough area to search. But, it's up to you to explore the zone and pinpoint the location of the barn yourself.

Sometimes this is easy, and sometimes it can really tricky--barns can be in plain sight or hidden within a dense forest. But finding them is worth it--the rare and iconic cars will be restored over time and gifted to you for free, adding a nice boost to your Car Collection level. If you're having trouble locating some of these Barn Finds, or just want to save yourself some time, this guide will help you find the exact location of the barns for particular cars.