Far Cry 5 > Prepper Stash Locations and Walkthroughs
Stash Mission Start:
There is a note at Sunrise Threshing.
Enter the garage:
Shoot the lock of the ladder at the silo. The ladder drops down and you can climb up the silo to use the zipline onto the garage roof.
From there you can get down to the garage little yard. Here you can shoot the lock to get outside. Get inside the red truck and attach the trailer to it to drive it away. The trailer is sitting on the bunker entrance. It will attach to the truck as soon as you bump into it. No need to exit the vehicle. Then you can open the bunker.
Inside go up the ladder on the other side - result. 3 perk magazines inside.