So i've largely gotten the interior rattles sorted other than the parcel shelf area.

How the eff does this mess all go together? I can't get the rear upper piece (sits in the very back, above the parcel shelf) to fit, and i think because it's not sitting where it needs to, it's allowing the entire parcel shelf to bounce and causing a racket. Anyone have tips? I'm going insane.

 

A quick fix would be to stuff some towels under it from the trunk

 

A pillow probably isn't going to help me fasten the plastics together.

 

The service manual has a couple pages that cover "rear tray". Maybe enough there to point you to the culprit.

 

Yep, the issue is that the "rear tray trim" seems to be (and i know it's not, but this is my perception after actual hours of trying) about 1" too long, making it seemingly impossible to fit properly between "left rear tray trim" and "right rear tray trim," then none of it fastens correctly, and the back of the rear tray itself is free to flop up and down and make a racket because the "rear tray trim" isn't tying all 4 pieces together as it should.

I've.... built a lot of cars, and i can't remember the last time something that looked so simple baffled me so hard.