At least as far as the desert Southwest is concerned, car temps will reach levels that will melt any candle. Everything I tried eventually became a shapeless blob with a wick.
Have you considered a candle made of
liquid paraffin? While they aren't the size of a tea candle, they last 100 hours and are for most intents and purposes smokeless.
Example
http://beprepared.com/product.asp?pn=CL%20C700It has a wider base to be more stable. Could be hot glued to the bottom of a small coffee can with a rubber snap lid. You could even fasten up your own handle to hang it by. Best option I'm aware of for an emergency candle in a car winter survival kit.