I don't understand how they could offer you any goodwill if in their opinion the fault isn't a manufacturing defect.
If the car is still under warranty then they should be paying for all of it or none at all.
They rejected the warranty claim saying it was caused by outside influences. Saying the car had not been washed enough. They are therefore offering this goodwill gesture which is a lot of pish.
The car has been serviced every year at a Kia dealer. I have spent a fortune with them