Honda Pioneer 1000 2017 Owner's Manual page 74

If the rear seat back has excessive vibration in the stored position or
if the doors are hard to close, adjust the rear seat stopper.
Rear seat stopper is located below the rear door opener handle.
1. Loosen the bolts.
2. Adjust the rear seat stopper in proper position.
3. Tighten the bolts.
bolts door opener handle
rear seat stopper