If your talking about the steel shelf that makes up the back deck, then I believe that is a piece that is integrally welded into the body structure, and I don't think you can remove it without a sawzall. You should be able to remove the light base from inside the trunk, it possibly snaps into the shelf or is attached with screws. I'm just guessing here but it would seem stupid that you would have to remove a piece of the body structure just to get that out. I replaced the contacts for the bulb in that light when I first got my car, but I can't rightly remember how the base itself was attached to the rear package shelf, probably because I didn't have to remove it. Why do you have to replace the whole base? If it's just that the light isn't working you can clean or replace the contacts, as I did, with the base in place.

Climb in the trunk and look around, it might be obvious once your in there.