My best guess is the car was at above freezing for long enough to keep the water liquid. Many mornings I have woken up with frozen nalgene water.

If you are camping overnight in snow shove the bottle upside down into a pile of snow. The snow gives a little insulation and the freezing will start at the bottom. In the morning turn the bottle right side up and hopefully you will have a of liquid water on "top" of the ice.

Cameron