In Norway, and rest of Scandiavia, DEFA heaters are widely used. They are known to be the best ones. You can read about it here.
They warm up the coolant that again heat up the block, oil, and give a little heat to the coupe. The coolant we use here is able to handle temperatures down to about -50C, so frozen coolant should not be a problem problem if you regulary check how much cold it can handle.
I have a Webasto heater in my Def110, and it is to good help, but it really drains the battery if used without the motor running. I use the Webasto only when I drive, cause the motor gives poor heathing to the coupe. Besides, the Webasto can be a pain to start if it's really cold.
So it's not unusual to have ice on the inside of the windows. Luckily the cars sent to Scandinavia have a Northern Climate Pack, with contains heated front and rear window, heated front seats and bigger alternator and battery.
I plan to put in a complete DEFA system in my own car soon. I have had it in my earlier cars, and it really is helpful.
I don't live in the coldest part of Scandinavia, but from time to time we have down to 30-35 below, and to get in the cold car at 6.oo in the morning is not just a pleasure. Last winter we had one night at 43,8 below. My 110 Td5 refused to start that morning. Even with the new 115 amp/h battery I had. But luckily I live on the top of a loooong hill, so I managed to rollstart it. But it was a very cold start of the day before I got to job.
Ronny S