I used my Jester as a year round vehicle and that had a fiesta heater in it but to be honest it didnt do much. Thick socks and a big jacket are the way to stay warm in these cars.
What you will need though is a demister. Im making a demister from an old 120mm computer water cooling radiator.
Ill make some simple ducting for it and plumb it in to the radiator lines.
Its kind of the same lines as these.
Cheers
Stot