best way to warm a minivan ?

[macphanatic]

I use what was called a space blanket (about 30 years ago). It's red on one side and silver on the other. I put the red side down with a foam pad and sleeping bag on top. The silver side reflects the heat up and helps prevent heat transfer to the ground. The blanket (more like a tarp) was also sold for survival purposes. Wrap yourself in it, red side out. Absorbs heat from the outside and helps keep you warm on the inside. Works quite well.

You could hang thick blankets along the walls of the van. It won't warm it up, but it should help keep the cold from radiating in.