"Stretch" wrote

Go to the furnace, shut the power off and disconnect the green wire
from the thermostat (the green wire connects to the G terminal and runs
the blower). Then turn the power back on. If the blower still runs,
the problem is not the thermostat or thermostat cable.

Not trying to question your knowledge there Stretch, but:

What if (and I have seen it happen many times) the wires got crossed
somewhere in the wall? Someone was hanging a picture and drove a nail
through the wire. Wire is run in the attic someplace and they set something
on or against it and pinched them together. Someone had the cover/door off
the furnace and the pinched the wire putting the cover back on.

What you describe could maybe eliminate the thermostat but not necessarily
the thermostat wire....