So here's a tip for those who may run into this in the future: if your heater goes out and its very cold out (i.e., middle of Canadian Prairies winter cold) the only way to see out of your windshield is to drive around with your windows down. Keeping the temperature the same inside & out keeps the windshield from freezing over inside.
A slight variation on this if your ventilation fan works but your heater doesn't: Adjust so the fan blows cold air on the window... Drove for two hours in -10C (5F) like that with my wife and kids wrapped up in the back seat. Not really a problem, but I was glad that the trip wasn't longer or that the weather wasn't colder. My wife and kids were fine, but the driver (me!) had to stick my forehead right into the cold airstream...
Being equipped for a weekend stay-over in winter (lots of warm sleeping bags and clothes) made the difference between a slight nuisance and two hours of hellish, brutal cold. We also had the option of waiting a few hours to catch public transport (train or bus) - which we probably would have done if our sleeping bags hadn't been available.