If you really want a warm cabin, why don't you get a 12 dcv ceramic heater that plugs into your lighter?

I've got one of these. We had our worst winter ever last year, and believe me... the little lady I was dating was in love with it.

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1981 300SD - 283,000 KM's at purchase, 360k+ now. Engine replaced at 311k. 16" CLK wheels, w126 gen II cosmetic upgrades, late w126 leather interior. RIP. Parted and gone due to fire

1987 300SDL - 243K miles and counting. In winter hibernation!

2001 Ford F250 Super Duty "Platinum Edition" Lariat 4x4 7.3L turbo diesel, 295k+ miles, various mods for reliability and performance.