Hey folks, just bought a 3.0 SC XF250 2015 model and its only running the basic sound system. Two questions please.

1) how many speakers does the basic sound system have? I am guessing no subwoofer as it lacks bass
2) how difficult is it to tap into the OEM basic sound system and simply add an underseat or low profile subwoofer in the boot? I dont want to go adding amps or changing speakers inside the car, just need to add some warmth / bass. Would tapping into two of the rear speakers give enough low frequency for a signal or is there a better way of doing it for this system?

Thank you.