My Dad put cardboard over the radiator of our family car during winters, back in the 50's. Just driving around town, the engine would not heat up eniough to make the heater work well. In the days of manual chokes, it also helped the engine get up to normal operating temps. a lot quicker.
I currently have half of the radiator on my 97 Powerstroke diesel covered up.
Sure helps get it warmed up quicker on these cold mornings.