The touring trim has a wire in the headliner for the xm antenna. Check out Schrute's journal,
schrute's journal, he tackled it and you can see his solution. He says he took apart the rear interior panels and I'm guessing he just pulled the headliner down from the back gently to access the wire.
My Z rattles there as well and I'm going to be installing the rear fog light and harness from the z-store in a few weeks, so I'll have to take the interior panels in the hatch out and I'll attempt to get to the wires in the headliner.
Sorry I can't be of more help.