Mazda BT-50 2012: Warranty claim

My 2012 Mazda BT-50 ute has had no front and taillights, and brake lights only on the left-hand side from new. Mazda say this was caused by the failure of the body control module and the reason given was the fitment of a non-genuine tow bar wiring harness. This equipment was fitted by a reputable tow bar company who say Mazda is dodging its obligation, as they will not give me any warranty claim on this part. I have contacted consumer affairs who say Mazda is treading a fine line in trying to prove the failure was caused by the harness and not a faulty BCM module in the first place. Cost to replace is over a thousand dollars. What is your opinion?