Posted: Jan 08 2009 at 8:45am | IP Logged

Hi and welcome.  A few things I would try that worked for me on vehicles with odd smells such mildew, smoke, ect.  Baking soda- an open box under the seat for a couple of weeks works wonders.  I would spread some on the carpets and let it set for a bit then vacuum it out.  It can't hurt it. 

   As far as leaks you actually need to see where it might be coming in and a simple test would be to be in it on a rainy day, go thru a carwash and watch for any incoming water/moisture? 

  Its also possible some prior owner left the windows open on a rainy day and there is some residual moisture.  I would check the carpets, under the seat, ect..

It might not hurt to change the cabin air filter.  You can get one off Courtesy Nissan Parts, type that in google and takes you to it.   They run about 27.00 to replace and they are located under the passenger side dash behind the glove box I believe.  A Haynes or Chiltons will help you better find it. 

Good luck.