Don't forget that all diesel models for a number of years now, have a built in 1kw electric heater built into the heater matrix. Whilst it won't help with defrosting much, it does get warm air out quicker.
When the climate temperature setting is above about 85% of maximum heat, and the ambient temperature is below (I think) 5°C, then the unit switches on automatically and heats up the cold coolant passing through it to not only give you warmer air in the cabin within minutes (which it does very effectively), but also help the engine warm up quicker.
The GTD still has one of these.