Speaking of electric blankets - check the archives.  I posted something
back a considerable time ago about a similar product - an AC/DC electric
blanket-type thing, only it is more like a mattress pad.  It cycles
on and off, and effectively averages about 4 amps per hour useage.  I've
used it - QUITE effective for something which plugs into your cigarette
lighter.  If you don't like the idea of leaving it plugged into yer
system all nite (like if you don't have an aux. battery), you can use
it to just pre-warm the bed, get in and zzzzzzzzzzz in luxury warmth.  As
I recall the company is called Electro-warmth, and is somewhere in the
midwest.  The things aren't cheap, but were the only thing I could find
on the market to meet that need.  Think I had to pay about $70, but
then regular electric blankets can be up there too, and they aren't
AC/DC (this is actually WIRED for both).