Stop messing around and take the whole tail OFF!
You are not going to KNOW if you have it "seated" in the "pocket" where the ball goes unless you have your eyes on it or know what it feels like when it pops in.
Seat off
Remove all black plastic pop rivets
Remove all bolts. You may need to "break" the factory locktit so prepare to hear a snap when you break free the bolts hold the back bolts and front baggage Allen bolts.
Carefully slide the plastic off until you feel the wire loom for the rear tail light lead.
Disconnect Tail Light Wire loom
Inspect your locking mechanism
Try with key to assure proper installment
Reinstall by reversing process
DO NOT OVER TORQUE!!Especially the vertically installed Phillips screws on the inside of the rubber grommets. They will strip if too tight and improperly removed.
I hope that helps.