Your water will thaw when you bring it into the cab of your car or truck. While you want at least a gallon of water, I like to keep it in quart- or liter-size bottles (usually from washing out and reusing bottles from G2 or similar drinks). I prefer candles or homemade paraffin heaters to sterno -- no poisons or leakage to worry about (although you don't want to leave them in the car when things get warm). Make sure you have waterproof matches or a lighter. Add a fleece blanket to your inventory -- doesn't take much room if you put it in a compressible stuff sack.

If you have to light anything in the car, be sure to crack the windows just a tiny bit to let exhaust gases out and fresh air in.