Looks like d4 aux heater is made by eberspacher not webasto - part number 4H0265081H for the heater itself, but then also would need the control unit, dosing pump and fuel lines, some wiring and remote.
http://www.ebay.co.uk/itm/orig-Audi-...-/222023457677
http://www.ebay.de/itm/Org-Audi-A6-A...UAAOSwx2dYHHNY
Shouldnt be too hard to retrofit assuming one really wanted one and had all the parts i reckon.