A pot of boiling water . . . ROFLMAO: all you would do is condense all the steam on everything in the garage that is cool, like the insulation backing, floor, tools.....and the car !!!
Seriously: where you live it will cost a LOT to heat the garage full-time . . . your car and possessions will rust unless the humidity is low . . . it won't even be good for your vehicles to keep them at, say, 50 degrees then drive them out in the morning when it's 10 below. Any kind of combustion heat creates humidity (even more than your pot of water) . . . . so your choices are to try and heat to a reasonable temp with electric heat (which will cost a fortune) or just warm when you want to work in there: a separate gas or propane heater (high efficiency with vented exhaust), or one of those kerosene bullet heaters that needs ventilation.