My car will not switch off?

I have a robin Hood kit car, which uses the electrics from a ford capri, problem is it will not switch off. |Starts fine runs well but when you turn the key to the off position and pull key out, the car continues to run, the only way of stopping it is to remove one of the coil leads. all electrics work , called the AA who thought it was the ignition switch , which I changed , still will not turn off, next thought was it had something to do with the alternator ? if you pulled the plug from the back of the alt , the key worked. he muttered something about an open circuit. so changed the alternator , started the car, but it still wont turn off !!!!!
Running out of ideas and money Help please