Open boot, remove plastic tray. There are 2 or 4 bolts on the back of the rear bulkhead. They hold the upper seat cushion in place. Remove them and with a bit of fiddling the back bottom cushion can be convinced to leave and then the upper back can be removed.
Of course just removing the plastic tray may reveal what's loose back there and may have nothing to do with the bulkhead.
Everything rattles and vibrates in this car. Have Loktite in hand.
