Holden Colorado 2017: Warranty

I'm currently battling with Holden on warranty for a faulty steering rack in my 2017 Holden Colorado, which I ordered from the dealer with a 2-inch lift kit, and is on its second going third new rack! The first rack was replaced at 18,000 km with, their words "some sort of internal fault", the cause of which the dealer could not explain. This was an approved warranty repair through Holden. I did not want a new rack and I asked for a new vehicle arguing it was a major fault (along with numerous other problems I'd had and still having). Holden denied my claim saying they did not class my rack failure as a major fault? That rack lasted until 40,000 km before it had the same issue. So the dealer once again set about replacing the rack with another new one, still no wiser as to what was causing the fault or what the actual fault is. Once fitted the new rack must be calibrated to the vehicle because it is electronic steering. The rack failed to calibrate to the vehicle prompting the dealer to ring Holden Tech who asked if the replacement had been approved through the warranty department. When the warranty dept. asked if any modifications had been done they told them there had been a lift installed. At that point Holden denied warranty on my repair sighting that they believed that the issue was caused by the lift kit. The dealer put my old faulty rack back in the vehicle and told me it was safe to drive but the issue (remember they don't know what the actual fault is?) would continue to get worse and would need replacing in time. I have this in writing. My argument is: The warranty was approved the first time, why not this time? Also, do they need to prove that a particular aftermarket problem is causing the issue to deny warranty? And finally, the aftermarket suspension was ordered as part of my dealer delivery. So is it not the responsibility of the dealer, that if a part they had fitted for me causes another part to fail that they must replace the said part? Where do I stand on this?