Cupra mate, I feel for you but would also need to warn you of an uphill battle with uncertain outcome. If you go Legal, then, they would ask you how long you had the car and when you noticed the cracks (the longer the period or, if you are not the 1st owner, the less your chances of success). They would also send an expert inspector to assess the situation. If the inspector finds scratches on your lights or in the surrounding lights area, then your probability of winning would be greatly diminished. Over tightening is a week argument as torque power tools are being used at the factory (at least this is what they will argue). Maybe you need to assess what is the legal bill you will be called upon to pay against the cost of replacing your tail lights. Or, try and negotiate with them once again if you reckon you may have a chance. Wish you the best of luck whatever course of action you decide to take.
I don't have this with my Cupra model as there is a ledge on the top of the boot lid for closing by hand. I only noticed this 'crack' after looking when my indicators were going really fast after a jet wash. I could see a crack in two places and condensation obviously shorting the lights. Its a year old so Im somewhat annoyed after a string of other issues, this is the final straw.
