Is it a factory installed stereo under factory warranty? Do you have any other electrical problems? My advice is to just stay calm, investigate everything you can about your truck and your state lemon law, and pray for this to go away. If it doesn't, at least you will be prepared for the enormous fight you will have with Nissan trying to get a new truck (provided it's a factory warrantied part). Good luck. Sorry to hear about your misfortune.

Matt