Right, forget the warranty, that's not going to help you here.
Your argument is with the supplying dealer. You pointed out a fault within the first three months of ownership, so it'll be presumed that the fault was there from the time you bought it. Take it back to them, and ask them again to repair it. If they won't then get that in writing, along with you asking them to do it and dates of exactly when you originally asked them. Get a copy of the Nissan tech document stating the fault and how to repair, and then get that repair done yourself. Take the bill and go to SCC, you have an easy case here under the SOGA.
Make sure you have all the correct paperwork though, especially with regards to dates.