My understanding is alot of the name brand("Battery Tender" for example) chargers are considered smart chargers, which charges untill the battery is full then switches into a monitor mode only charging if it drops again.
I have a little Battery Tender for the motorcycles. I use it on the truck and the Mercedes if i let them sit. Wiring one into the heater plug is a great idea!! I don't have the experiance with brutal cold like you guys up north but seems like a awesome idea! And these darn diesel batteries are not cheap!!!

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2006 Jetta TDI DSG 320k miles
1997 Ford F150 325k miles 4.2L V6 "Work Truck"
2008 Tundra 225k miles 5.7L
1982 240D.....sold
1984 300D...Totaled OUCH!
1985 300D Turbo 222k miles "Dos" sold to 79Mercy
1986 300SDL 98K miles "The Beater"....sold
1987 190E 2.3 16v Euro spec 115K miles....sold