Then recommended 10 minutes doesn't have to be in one block.

I do a lot of waiting in winter, so I turn the heat up high, the blowers on high, and leave them there. I heat up the car until it's really warm, then turn it off. I don't run it for 10 minutes and then suffer for 45.

Cars just aren't insulated very well, and you don't see many double-paned car windows. I carry several cheap mylar blankets, and if necessary, I would roll the edges up in the windows and drape them down the car doors. This would probably work even better with several people on board.

Sue