So a couple issues, but its definitely been done by others- there is no line level outputs, sub or otherwise (that I am aware of). The interconnect from the headunit to the factory amp is fiber. I dont know of anyone that has tried to see if its toslink, but that would be a much bigger project anyway. You will have to tap the factory SUB speaker wires for input. Either under the passenger seat, in the tailgate, or some where in between. I have the pin numbers of the amp connection at home if that would help. You need to be sure your amp has line level inputs and not just RCA jacks.
My tailgate 'sub' in the SE definitely sounds more like a full range speaker, so I doubt there is any crossover in play. Sorry for the bad news, but I hope it helps. Be sure to post some pics once you get it figured out!