Sue, I'm going to say I've never tried, but here are my thoughts.
Clothes, sleeping blankets, snow caves all work by trapping body heat. The loftier they are, the better. That being said, I think it would probably be warmer to keep the blanket on you, and create a small zone of trapped air, than to insulate the entire car and then be trying to heat up that air with your body.
That being said, I suppose if the car was full of passengers, then you might be a bit better off sharing collective body heat.
Thank goodness the mylar space blankets are cheap, and most folks can afford to carry more than one, in case you want to experiment.
+1 on the smaller area. I took a class once for "winter driving survival" to get out of a bad detail in the Army. They taught that if you use whatever is available to section off the interior of the car to only the area needed, you had less area to heat.
We were using a CUCV (old Chevy Blazer) and used blankets and 100 mph tape to curtain off the cargo area. Made the passenger area noticeably warmer.
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