For real? You leave it run. I was considering this today, didn't get above 10, but I tried it the last freezing rain storm we got and got told about it.
I was having problems with the fuel gelling yesterday, so if the distance between 2 stops was very little, I had to let it idle or I had problems starting the truck. I only had to do it 4 times though, after that, it seemed to warm things up enough that it quit jelling in the filter.
I usually dress warm enough that I can leave the pass door open when I am doing in town resis. A bunch of years ago, my heater quit working on a -5 day. By the time I got to lunch, I was cold to the bone. I stopped at my lunch spot and the owner let me eat in her office so I could take off my layers and warm up my clothes. She also refilled my water bottles with hot water so I could finish my day.
The coldest day that I worked was -22 when we left the building and the high for the day was -15. Left 3 blank info notices because my pens froze up.