Thanks Sean,
But I think this one has gone well beyond the basics now. The heater is not just a continous drive on this car, it is PWM driven by the PCM.
I can knife-and-fork it but a circuit diagram would save me searching down every pin on the PCM and would enable me to check where the supply is coming from. What makes testing even harder is that the multiplug is under the car floor and is not accessible from under the bonnet.
Regards Shaun.