I had trouble with mine coming loose and I replaced the nut with a thin nyloc nut. I believe the shoulder bolt threaded portion isn't very long so the lock nut has to be thin to catch the threads. I'll look tonight and see if I used a thin nylock nut or an all metal lock nut. It helps to be a contortionist to get to these from the nose of the car. Not easy to do.
It might be the same bolt used on the trunk but not 100% sure. If you email I'll send photo of mine.
mvanhorn8893@gmail.com