I think the Miller CMF Series 80-PG Gas Furnace is around 20 years old. I need to know about the ground wire to the blower motor. Does it go to the door switch or to the heat limit switch?

The white wire from the blower motor needs to be connected to the white wire from the house wiring perhaps at the door switch. The green wire from the blower motor needs to be connected to the bare wire from the house wiring, perhaps by both being connected to the metal frame of the furnace. The white is really called a neutral, not ground.

It won't go to the door switch since that should be 120V hot not neutral.