I would consider the "heater box" to also contain the evaporator core and the mount for the blower motor. It's more of a generic term for the large chunk of plastic inside the cabin, under the instrument cluster, that contains the large interior HVAC components.
But, keep in mind that my experiences are general, I have no specific experience with the Yaris to know if Toyota did something atypical with that HVAC system.
-Rod