My tiny diesel car (2008 Toyota) needs its auxiliary heater below around -15 C for highway trips. It's a switch in my dash that burns extra fuel, otherwise the engine won't get up to or stay at temperate.
Pretty normal with diesel as it gives off less heat than petrol. I have a van with an 88kW engine, and even at -5c I can see the coolant temperature drop when I am idling down hill and have the heater on. Any colder and it's worth blocking the radiator with cardboard.