I think most vehicles have some degree of insulation in the doors and roof for soundproofing. Covering the windows with reflective bubble wrap could very well be worthwhile.

I've been experimenting with a semi-transparent foam-plus-vapour-barrier material used as underlayment under basement subfloors. It comes in huge rolls, weighs nothing, and adds more insulation than you'd expect. Perfect for temporary winter windows in my house during renovations. I think it would be highly useful if stranded in a car. Similar to this: http://www.homedepot.ca/webapp/wcs/store...tk=P_PartNumber