a lot... I baught a digital piece out of a 240sx thinking I would be able to do it, basically there are more wires coming OUT of this thing then I have in my console... Also dont forget the sentra is manual... look behind it, its got those sliding bars and shit, you'ld have to get the motors from the G20 at the same time of purchase. basically you'll need a professional to install it.
yeah, it's a lot of work. But your best bet is to get a digital climate control system built for the Sentra just like what i have in my ride. too bad you have to import it from Taiwan or the Philippines to get one.