You can get heat using the A/C flaps, kind of, sort of. You can not pull the air through the heater core nor push it through since it vents back out through the defrost motor, what is actually its inlet. What you can do is, open a rear window just a fraction, turn the heater levers to heat, flip the changeover switch to the A/C position, undo the clip holding the flap to the vacuum changeover diaphragm, flip the diaphragm to the "heat" position and use the A/C fan on "low" or "medium" to move the air being forced through the heater core by the flow from the open rear window into the A/C ducts and back into the car.

It works but its not exactly efficient. It'll defrost the windows and keep away frostbite. On a long or fast drive it will actually make the car semi-comfortable. Except for the poor sap sitting beside the opened window.