I did the 12v heater thing last year. And it sucked for a few reasons I wont get into here.
The battery in the trunk would be useless. Standby mode is fine, but not all motherboards cut power to the usb devices, and those can drain your battery in a matter of hours. I use stand by, but have to use hibernate in the winter because my battery cannot take it. I have a yellow top optima (deep cycle battery) and I still only get about 48 hours of standby power. The cigarette lighter idea is also no good because of the draw that would be required to charge the battery would probably blow the fuse.
I would recommend getting this http://store.mp3car.com/2_5_inch_Sea..._p/hrd-010.htm Its a automotive HD. I am using it now instead of my Mtron SSD. Its rated down to -30c.