What Benno said... start getting all the evidence in writing and try and be nice to your Dealer and keep him on your side... he's the meat in the sandwich.
Essentially he has to get approval from Nissan to pay for the repairs so he has to be convincing 'cause if Nissan say "No" he just shrugs his shoulders and he doesn't get any money for fixing it and you have a poo fight on your hands.
Uuuuummm... I assume you have had the vehicle since new? Was it serviced by Nissan franchises?