'But I think this one has gone well beyond the basics now.'
Ok, but if you haven't even tried the basic's yet how do we know this?

'The heater is not just a continous drive on this car, it is PWM driven by the PCM.'
Yes it is,again a very basic system to check.

You are looking for a wiring diagram which is fair enought, but what I was saying was you can cut the possible causes of this fault in more than half in under 10 mins (including access underneath). This would put you much closer to a repair than you are now and you would have some facts to report to the owner at least.
So instead of asking someone to take 10/15 mins out of their day to help you fix this car why not spend 10/15 mins of your own time and narrow the fault down for yourself?

Regards
Sean