My heater is blowing hot air even when the dial is on cold any ideas why this would be ?.

 

The temp switch alters the position of a valve which allows more or less hot water from the cooling system to go to the heat exchanger under the dash.
Either this valve is stuck open or the switch/ wiring controlling it is b****ered. Not sure if there are any resistors/ fuses/ relays involved in the circuitry. You would need a wiring diagram to figure all the possible places for failure.