There is a cable on the back of the heater selector. This is quite hard to get to, the easiest way to get to it is to take out the radio and put your hand behind the switch as you turn the selector a cable moves a flap inside the heater consul. If its stuck then the flap wont move and you will get cold air only. However if you force it over to make the car hot there is a good chance you will brake the switch, fairly cheap at a scrap yard.
Andy davey, December 2007