A little research shows there is no cabin air filter on an ND, but something called a dust-free filter. Apparently it is a pain to get to. You should also have several drains coming off the top. On NBs there is one at the front of each door, and one just behind the seats where the rain rail drains into. Not sure exactly where they are on the ND but you should be able to find them.
You could also try removing your HVAC fan and see if any rodents have piled stuff in it. That will cause mustiness too.