Complaint

I purchased a set of 4 good year duragrip tyres for my corsa utility early last year. During the year I travelled approximately 6000 to 7000 km. Towards the end of last year November, the tyre on the rear left developed 3 cracks on the inner wall. I thought it could have not been normal as never drive rough. However I changed the tyre at my cost. Two days the second tyre failed on the opposite side with similar cracks. I then contacted good year South Africa who requested I contact their local representative. After calling him several times and sending pictures, he claims I ran the tyre under inflated which is totally untrue. My tyre pressures are checked once a week and never under or overflated. My concern is that from the failures it is quite obvious the front two will fail shortly and good year does not seem to take this seriously where my life could be in danger if those tyres burst. Can someone senior contact me as I would want those to be checked with an assurance the similar thing will not happen. I would want to my tyres to be replaced.