[brucepick]

Another way is to let the air intake suck in air that's been through the radiator.

On my car the ducting to pull air from any part of the exhaust system would be a bit tricky. So because the air filter box is located pretty much right behind the radiator, I cut holes in it and blocked off the snorkel. These days it's pulling in air about 35-40 degrees F warmer than outside temperature. That will vary depending on how much grill block you have and other factors but it's doable.

More grill block makes the radiator-heated air warmer. Best results in my car were with the lower grill mostly blocked and the upper left open - because my revised air intake is behind the upper grill area. So I want air to go thru there and get warmed by the radiator.