This stash is found in Sunrise Threshing as shown above. When you reach the location, you may encounter peggies. Kill them then read the Prepper note on top of a container near the tree by the road.
Find the zipline connnected from the rooftop of the garage to the nearby grain silo. Find the ladder behind it and shoot the lock to lower the ladder. Climb up and use the zipline to reach the rooftop.
The bunker’s hatch is under a small flatbed trailer. You can forcibly remove this by blowing it up with an explosive. Open the hatch then go down the bunker. Take whatever you want inside then head to the other ladder connected to the garage. The mission will be complete, with the unique truck inside as your prize, as well as a garage permanently unlocked in this location.