Heh, same problem here. Control box of the aux heater has gone through some kind off hellfire and was totally damaged. I replaced the whole heater with similar from Fiat Ducato, all product codes are the same. Radio is working and other stuff in the convinience bus but heater it self can´t be connected via VCDS. And also the heating timing seems to be somehow inaproprietally controlled, heating occured quite randomly instantly after installation and now later the heater has been very quiet. Is it so that similar heaters with same product codes can be different according to the original installation car/brand?
Common issue on phaeton's that came with auxiliary heating. The way that system is designed is if one modules goes down the rest of the modules on on that system will go down as well las one module send the signal to the next.
Can you address 018 Aux Heating? Based on my 2006 Touareg experience you will not get the proper response from the Fiat module. Get the exact replacement part for your VIN.