As for locating it on the decklid, I've never done it before, but I'd suggest measuring the distance between the rear outboard studs on the rack and checking to make sure it is the same as the distance between the center of the cutouts on the underside of the trunk lid.

As long as it's the same (I think it should be), drill your first two holes in the center of those cutouts.

Then, with some masking tape on your trunk lid to protect the paint, position the luggage rack on the trunk lid. Take some measurements to double check that it's centered where you want it. Now you can use it to mark on the tape, where the other four holes for the rivnuts need to be.

Are those rivnuts the same as the ones used for side mirrors? I think they are.

How are those holes suposed to be sealed so that water doesn't leak into your trunk? Perhaps some kind of sealer on the rivnuts when they are installed?

Tav