Second trunk light - how to install?

My 2007 A4 cabriolet has only one trunk light (in the rear passenger corner). At night it barely lights up that corner of the trunk. There's a space for another one, with a plug in it, in the back (inside the trunk near the center behind the rear bumper). I need to put a light in there, so I got the lamp assembly from the dealer parts folks, and am trying to install it myself.
 
I can't figure out how to get the black plastic cover off the inside directly behind the bumper. This is the cover that has the hole in it for the trunk latch receiver. I tried getting the side liners out too, but they must have some mystery attachment as well. I'm pretty mechanical, but can't seem to find the magic screws for this one.
 
Any clues?
 
Thanks,
Dan