Very, very unlikely that there's anything wrong with the wiring harness if it was changed 4 or 5 years ago. You have a really bad dealer there - one of the worst stories I've ever heard. You need to find a good independant shop or dig into it yourself. You can read the fault codes yourself on the 38 pin diagnostic connector in the engine compartment. Does the car have ASR? Is there an ASR warning light on instrument panel? Is the light on?