I've been paying more attention to my heater controls, and have discovered a couple of new (to me) things. This is all with a car with A/C and manual transmission.
First, if the heater is off then you can get autostop in cases where you can't get it even in econ mode. It seems to be right around 40 degrees (f) when this happens.
Also, if you turn the heater off and then press the fan button, the display shows the fan speed and mode and temperature, but it's not in auto and not in econ, either.
And if you're coasting to a light, even below the speed that autostop normally engages, but then turn the heater off, it will then go into autostop. If you wait until you stop, autostop won't engage.
Then when you press the fan button you get back to the settings you had before.
So right now during my commute I'm running in foot-plus-panel mode, 67 degrees, one fan mark. I coast up to lights, then press OFF at about 10 mph to get autostop. Then after I stop I press the fan button and the heat comes back on, while the engine stays off.
Incidently I did take out the defroster fuse which may have made a difference; it's hard to know in this weather...