I never had to try it. As another poster said, it's not going to make you toasty warm -- at 20 or 30 below, not much is. But it can provide the difference between losing some digits or not.

I never worried about setting anything on fire in the car. Keep the flame far enough away you can put your hand on the liner or windshield and keep it there. We carried votive candles -- short and contained -- with the idea of putting them on the console between the seats. You also need to keep in mind that the candle can burn down to the stub and set the car on fire if it's not contained -- you read about people setting their houses on fire letting candles burn down on their night table after they fell asleep. So yes, safety still applies.

Remember a burning car will keep you warm only on one side. So turn about is fair play.