I did a test with tea lights in my snowbound car. Four tea lights in a safe container, with their aluminum holders touching to ensure all the wax would melt, raised the inside temp. to -7C with the outside temp. at -14C and a brisk wind (-23C windchill). This was done without a person inside and the windows were closed (though the window/door seals are pretty old; I could feel a bit of air coming in). The car had about 3-4 inches of snow on top, and I shovelled snow to close the air gap all around the bottom perimeter. So there you have it: candles actually help.