I guess it depends on how badly this is damaged as to whether it can still be repaired in situ or requires being replaced. Regardless it will be a welding and spray painting job if it was to be replaced. Strangely these panels seem to spontaneously start to rust at the top edge and then downwards. I've seen quite a few like that over the years along with my eldest sons Acclaim which he had that panel replaced by a crash shop.
With the taillight removed, can't you get behind that small panel?
Mr cutting up a couple of sedans, I do remember that panel is spot welded on. It could be removed with some power tools like a recip saw and cut out some of the rear quarter panel.