The left attachment point of the rear wing on my car has become loose (must be all that downforce

). I've pulled the rubber plug on the inside of the trunk lid, and can see (and finger-tighten) the nut, but I can't get a deep socket into the hole to really tighten it (the hole, which is too small a diameter to admit the socket, is in a plastic part that appears to be riveted to the trunk lid liner). Anyone have any ideas?