Thanks for the input guys!
The part is
Support Cylinder Ball Stud.
I did some more investigation to see if more parts were rusting in my trunk. The nut & washer that holds down the tail light assembly had a little bit of rust showing, so I took it off (this seems like a weird spot to be rusting).
This is what i found
Took out the tail light completely, and no other obvious rust spots.
I do find it odd that there are spots of rust right under where the weatherstripping would be. Again is this normal? Seems like water is somehow "climbing" up the internal part of the stripping?
Hmm a leak is concerning. Where could it be leaking from? Since there's weatherstripping I don't see how there can be much of a leak..