Where the tonneau cover connects, on the left top, the clip that the screw goes into the hatch. I own 3 z's, 2+2, 2+0s, jdm, usdm, whatever. They are all broken there. This one is the best one I found which is cracked, but not all the way. All my other ones are in two pieces (including every local member i've remembered to have a look on their car).
It's a little obvious a screw goes there, not some sort of hidden clip. WTF.