I don't have a puppy in this race, because I just put gallon water jugs and LET them freeze. But A Goldenrod dehumidifier pulls something like 12 watts and heats to 150 degrees. It's not a source for fire, but I'd imagine you'll want to protect it somehow. Folks put a core heater in their cars for icy mornings, why not put a goldenrod in your truck amongst your BOB bags? Cover them with a couple of blankets and the water should stay liquid.

As far as insulating things. The issue is ALWAYS heat, either it's in a heatsink (like water) or it's ambient. You are trying to keep your heatsink (water) from releasing the heat to the ambient. Without active involvement you have a win/lose situation. You WANT ambient to bathe your heatsink when you're warm (like when the car is running) but you want to insulate it when ambient falls below freezing, either way takes active intervention (like uncovering it when the car is running). I'd put a goldenrod in a plywood enclosure and pop it in my trunk, plug it in at night. Wouldn't kill your electric bill and wouldn't catch things on fire.

But remember, the more liquid water you have, the more cold it takes to convert it to frozen. So a gallon water jug takes more to freeze than a quart one, and gallon water jugs are very flexible - if they freeze they just expand to fit the water.