Mazda 3 2007: Radio failure

The radio in our 2007 Mazda 3 Neo failed in December last year with two weeks left on the warranty. It was fixed under warranty, but failed again six weeks later failed again. It was booked in to be fixed, but came back to life before it went to the dealer. The dealer claimed they could find no fault, but it has recently failed again and Mazda is saying that we may have to pay costs as the warranty has expired. What claims do we have since this is a reoccurring fault from the warranty period?