Hello Lee some thoughts , the fact that you can communicate with the aux heater I would say it is powered up as the self diagnostic function comes from the aux heater unit itself .
The unit like already said is beneath the drivers side head light behind the front bumper , it's a bumper off job to get anywhere near it and it's still awkward to access the wiring with bumper off , when I bought my 4.2 tdi fitting the aux heater extra water pump was one of my first jobs I did on it , there's a superb thread on the subject called 'definitive guide to aux heater mod ' or something like that , it has all pictures and write up , it's done on a newish looking car , try the same job with 10 years of corrosion on the bolts !!
Run through the output tests with vcds and listen carefully near the headlight , is there and output test for ' diesel supply pump or maybe dosing pump ' for that one sit in the car and listen , it's a very faint ..... technical term ....kerchug , kerchug sound . Is there an output test for glow plug ??
My aux heater works fine but I have always had this code since the coding , as you can see there is no date and time stamp , perhaps the aux heater does not support it.