Nissan's refusal to replace New defective Ignition Key.

I purchased a New NP300 vehicle on the 15/09/2017 for R169000.00 Cash. On the 28/06/2018 which was my first service , I handed my Ignition key over which I had reported as faulty 3 months previously. The Dealership in Parow Cape Town refused to replace it. I then contacted Nissan HQ in Pretoria *** (case Number ***6) and after numerous Emails and telephonic conversations , they too refused to replace my defective Ignition Key. The key gave me problems from the beginning , I can't understand why they do not admit it is Defective . I paid Cash for the Vehicle and have it serviced by them. I should have purchased a Toyota .Your assistance in this matter shall be greatly appreciated .Kind regards Russell Sevenster ***