I lived in northeastern Ohio when I was a college student, 1981-85. Driving into Cleveland or wherever during the winter in my buddy's dying Buick Centurion (gosh, do I miss that monster!), my friend and I constantly worried that the engine would turn over one last time and leave us stranded in a snow storm, or something apocalyptic like that.
Along with the shovels, wool blankets, etc, in the trunk, we kept a box of big ziplock bags in preparation for the Immanent Breakdown. We also kept a box of those otter pops, good frozen or in liquid form, available in any 7-11 worth its reputation as junk food heaven.
If we ever needed water, we would simply pack some snow into the ziplocks. In the car cabin (or any warm environment), they would thaw very quickly. Nifty for First Aid coldpacks, too. Snow outside of a city, retrieved from somewhere other than immediately by the side of a highway, should be pretty safe; but potable aqua tablets is probably a good idea, anyway.
Of course, I suppose you could also simply freeze water in half-filled ziplocks beforehand and stack them in a box or milk crate in the trunk. They would certainly melt faster in this form than in a one-gallon plastic jug. You could thaw them immediately in a pot, if you make a habit of carrying some water with you anyway. In fact, those nifty, triple-lined Platypus water bottles that Rigormootis mentioned are a worthwhile upgrade, available at REI, Campmor, etc. They are designed to withstand freezing temps as well as boiling.
If desperate (file under last-ditch effort), one could sponge condensation off from the inside car windows -- ideally, with a sponge, but a handkerchief, t-shirt, etc, would work, too. Wipe and squeeze into a cup. You will, of course, also end up absorbing whatever else is on the inside of your windows (dirt, cleaning solution, etc.). Which is why keeping water, of course, is a good idea.
My two cents --
Glen
EDIT: Oop, just noticed that Rigormootis was referring to the Nalgene hard bottles. Those are great, too (what I carry daily). But I was referring to the soft Platypus bottles, that flatten as they empty. Again, they are designed for freezing and boiling, and are also pretty puncture-resistant (far more so than ziplocks).