I'm in ND, too, and I'm building my winter car kit right now, too. We haven't had a ton of snow in past years, so I thought it might not be a bad idea to keep some water in the car, too, in case I get stranded somewhere without any snow to melt. The problem with this is, I'm thinking if I just put a jug of water in the car, it'll freeze and burst. One thing I thought of is wrapping my blanket around the water and putting it in my container to insulate it, but then if it does burst, my blanket will be all wet and useless to me. I know I can get those little water pouches, but I really don't have the spare cash to do that right now and I'd rather do this with an 85 cent jug of water instead.
Also, what all did you put in your kit? I always feel like I either have way too little or not nearly enough. I need some help finding my happy medium. 
Cold is cold. Wrapping it in a blanket will slow the water down from getting cold, but it will eventually chill to the 'average' temperature of the car. If it's ten below out, it's going to be ten below inside the car way quicker then you or I want. The blanket will only slow this a little bit.
i have a couple of small bottles of water in my vehicle, probably half a dozen small bottles of water. I get ones that have a good sized air bubble on the top - so the water has some room to expand to. I also get the ones with the heavier bottles, because that air will compress as the ice moves in on it's space. Try putting the bottle into a dish and pop them in your freezer. See if it bursts open. I haven't had any of trouble with the water bottles I freeze to keep the coolers cold. If a small water bottle bursts, you only have a little mess in the car till spring, if a big water jug bursts, you have a big mess until spring.
What you keep in your car is really dependent on where you drive. If you drive from home on one side of Fargo to work on the other side of town, you won't need near the stuff you would need if you live in a small town and need to drive 50 miles to the 'city'.
What's the worst that can happen? Slide off the road and get stuck driving home some night from the city getting supplies for the blizzard that is blowing in?
You would need things to keep you warm. Keep a set of mittens, hats, heavy socks, and blankets for everybody in the family in each car. They can be old shabby ones. Kids are cold enough, the won't care if they aren't stylin'. If your alone, four blankets is a lot warmer then one.
You would need things to get the car unstuck if possible. A tip, a tow-truck or a local's tractor is GREAT for getting a vehicle out of the ditch. If you drive through areas with no cell phone reception, a CB radio may be in order. If they build the ditches in the Dakota's like the do in Wisconsin a shovel and non-clumping cat litter or a tube of sand may not help all that much.
If you feel you need to walk out - unless you can see a place or know exactly where a place is, don't. A car is a lot easier to find then a body. A plow truck will know when it found a car buried under the snow - it may not notice a body under the snow.
Have that map and compass of the area you are walking through. I haven't been through the Dakotas in the winter, but I remember a portion of it being mighty the same in the summer.
