Poor service at Mercedes Benz South Africa

My vehicle is on a motor plan. I took it in to a dealer to fix another problem and they raised that my brake pads need to be replaced. They then informed me that Mercedes Benz is refusing to pay for the brake pads due to 'excessive use' as the break pads lasted for less than 18 000km. I have asked for the contract or motor plan that specifies this limitation. They have failed to provide this. There is no such clause in the contract provided to me when I purchased the vehicle. They have also failed to respond to my many messages and emails. Very poor service.