Refusal To Repair Vehicle

I took my Vehicle in to Mazda Polokwane to be repaired after being referred there by Matpoi Motors Lephalale. Both dealer ships have said the rear Diff in my vehicle needs to be replaced as there is internal damage. Innovation Group are the holders of my extended Warranty with Ford Protect they say they will not replace the Diff as it costs too much. This is not the only problem i have experienced with them three months ago the told me that my policy had expired when in fact i still had six months left on the policy. The fact is a warranty is there so that if something is defective on your vehicle due to mechanical failure it will be replaced. I get the feeling they are now playing for time so that the policy will expire then it is no longer their problem but it will then become mine. They are also second guessing Mazda as they say they see no problem with the Diff but two Mazda Dealers have said it needs to be replaced. When you phone them to sort out the problem they say they don't deal with you they only deal with Agents.