I have to agree with Kaiser on this.

Do NOT mess with anything yourself. IMO. the dealer needs to find the short, even if it means removing the dash and completely replacing the wiring harness that supports the audio system. If after that there is still a short and a problem then they need to either buy back the truck or replace it. Even if it is fixed, I would try and get them to give you an extended warranty or some other sort of compensation. If they fail to fix and won't refund or exchange then talk to an attorney about the lemon law. In many cases, one or two calls from an attorney (a good one!) to the dealer and/or Nissan will light a fire under their asses and it won't take too long to get a resolution.

Also, if you haven't already done so, you can try calling Nissan COnsumer Affairs and see if they will help you out at all. They are notorious for not being very helpful, but in some cases they have...it probably depends a lot on who you talk to.