Dagny, my experience with a Heatsheet and fleece sleeping bag was on the near side of miserable. The evaporate just soaked the bag because the Heatsheet kept it in. Terrible. Now, in a car it may be different because you may be sitting up, which helps the "steam" escape. But, lying down, terrible.
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"Its not a matter of being ready as it is being prepared" -- B. E. J. Taylor