I thought about this the other day while I was sitting in my car in a parking lot with the heater on full blast just to keep the temperature at a stable level.

With the heater putting out a much smaller amount of heat, it got cold pretty darn quick. Not as cold as it was outside the car, but cold.

I suspect any kind of small heater like a can of Sterno or a small alcohol stove are not going to change the temperature very much inside the car. A few degrees one way or the other won't make any real difference.

Better to save the fuel to heat water to warm yourself up from the inside out IMO, and get some hand warmers to stuff in your boots and inside your clothing to warm you up from the outside.