There is a flanged area that the rear window sits upon (factory black painted area in photos below). The area where the rear quarters mate up to the rear bulkhead leaves a 1/8" drop that is factory filled with dum-dum sealant. (the area just to the right of the screw hole in the photo below)
The filler panel mates up to the edge of that flange and was installed prior to the window being installed so it is usually stuck in place by the oozed out window butyl sealant. The upper right slotted area is where the screw goes.
