[ijames]

If you are sitting in a car you are out of the wind, so that's the biggest win right there. Get one of those jump packs with a built in inverter, and a heating pad, and put the heating pad on the seat back before you sit on it. I don't think you need a full heating blanket, judging by how nice my heated seats feel in the winter even on low. Mine heat either just the back or the back and bottom, and for me heating just the back is plenty. Don't worry about the inefficiency of going to 110 VAC for the heater instead of getting a 12 V heater - any power "wasted" in that step just warms the air up a little in the car, so put the battery pack on the floor in front of your seat so the heat rises onto your legs. My heating pad is 50 W on high, and once it warms up I have to turn it to low if I'm laying on it, so a 100 W inverter is plenty. Use the battery pack so there is no risk of killing the car battery. Then toss in a little car blanket to cover yourself with, and you should be fine.