Warranty not paying!

I took my car tor Ford Middelburg because the temperature light came on while aircon is on.. After a day or 2 they told me they can't find anything. They advised me to bring the car back immediately when it happens again. So I did.. Again they told me they can't find anything wrong. I asked them to give me a letter stating they can't find anything. 2 weeks ago, I started having problems with my engine and the car overheats. I took it back immediately. According to Ford, 'someone' worked on my car and the plug/cable from the fan was not in place and got stuck between the blades. Ford says it is not their fault and the warranty refuse to pay..