I have really enjoyed this thread. Thanks, everybody!
I wonder if you might try taking some old carpet sections and laying them on the outside of your windshields, rubber backing against the glass, at least for night time when your view may not be that great anyway. I bet you might be surprised at the improvement in inside temps. And it won't have an energy cost for you, either.
Though we are located in central old Mexico, we are at 6,300 feet above sea level (1,000 feet less than Mexico City), so it is getting pretty chili here. Our homes have no central heating or A/C, of course. The last few days it has been so cold a man would snuggle up to his ex mother-in law.
Check out our weather conditions live here:http://weather.yahoo.com/mexico/queretaro-de-arteaga/san-juan-del-r%C3%ADo-143928/