Unfairly billed for diagnostics of my vehicle

I took in my car to KIA The Glen last Thursday because I discovered that the entire floorboard of my Cerato Koup was soaking wet. Initially I thought something had spilled and I left it to dry but the next day the water had accumulated and it was quite visibly increasing. They have since been trying to identify the problem with no success. I have been in constant communication with the workshop manager Eugene who has been trying to investigate the cause of the leakage but he, together with his technicians, does not know what is causing the leak. I was asked to leave the car with them for a couple of days. Each time I called them I was told that they cannot find the cause of the leakage. Today I was notified that they still haven't found the problem and because of that, I am held liable for the costs of labour which is over R3000 which I will not pay because it is entirely not my fault that they cannot identify the problem. They came up to the conclusion that I lodged my vehicle in water which is completely untrue. The only cost that I was told was diagnostics fee of R682 which I would pay if warrantee does not pay. I can't pay for a problem that was not found. I want my car back.