Affordable (<$100) AC power inverters that will give you 120 VAC power are available rated up to 400 Watts, although my cheap 400 Watt one can only drive 150 Watts reliably. Still, this should be enough to provide a little directional light. Put diffusion and Color Temperature Blue gels on the light. You could also try covering one or two windows on one side of behind the camera to see if it helps give some depth to the actors' faces. If you have access to enough Neutral Density gel, you might want to line the outside of the windows behind the actors so the windows won't blow out as harshly vs. the actors.
Both the car's heater and A/C will reduce the fogging on the windows, so don't use those. Just spread a bunch of wet towels around inside. Also, if you have an AC inverter, a humidifier that sprays a cool mist might help, and you can aim it at the window you care about.
