I have a first generation, but it did not come with the MACH system (so no stock subwoofer).
However, I did fiberglass my own enclosure into the stock location.
From my experience, I would say that it is very unlikely a 12" sub will fit there.
I am running a 10" subwoofer and it is a very tight fit. I have about .5 cubic ft of airspace. I'd recommend a 10" or an 8."
Selecting a subwoofer for this location is really limited by mounting depth, magnet and basket size/shape. Also, a 12" simply wouldn't fit behind the grille, so you would probably have to cut up the interior panel.
Sorry for that news, but I'm sure you can find a great 8" or 10" that would work well for cheap.
Or, prove me wrong. I don't have the stock enclosure, so I'm not 100% sure. Maybe you can figure out a way once you get a good look at it.
Either way, good luck.