An automotive heating, ventilating and air conditioning system (10, 54) comprising: a housing assembly encasing an air conditioning evaporator and a heater core; an upper mode case configured to discharge a first stream of temperature-conditioned air into a front passenger zone through at least one zone outlet; a lower mode case configured to discharge a second stream of temperature-conditioned air into a rear passenger zone through at least one zone outlet, said housing assembly configured to direct system inlet air through said evaporator and to trifurcate said inlet air into a first cold air stream directed into a first mixing chamber disposed in said upper mode case, a second cold air stream directed into a second mixing chamber disposed in said lower mode case, and a third cold air stream directed through said heater core; an upper temperature blend door operable to proportionally control the ratio of a first hot air stream exiting said heater core with said first cold air stream as they enter said first mixing chamber; a lower temperature blend door operable to proportionally control the ratio of a second hot air stream exiting said heater core with said second cold air stream as they enter said second mixing chamber; an upper zone mode valve operable to selectively obstruct said discharge of said first stream of temperature-conditioned air into said front passenger zone; a lower zone mode valve operable to selectively obstruct said discharge of said second stream of temperature-conditioned air into said rear passenger zone; characterized by a control system configured for controlling the automotive heating, ventilating and air conditioning system (10, 54) according to the method of claim 7, said control system comprising a driver circuit operative to selectively independently position each temperature blend door (48, 50) and each zone mode valve (39, 52), said control system further comprising microprocessor HVAC module mathematic model software operative to receive operator selected discharge temperature, discharge flow rate and discharge mode inputs for each outlet and to generate a separate drive control signal for each as a function thereof. a housing assembly encasing an air conditioning evaporator and a heater core; an upper mode case configured to discharge a first stream of temperature-conditioned air into a front passenger zone through at least one zone outlet; a lower mode case configured to discharge a second stream of temperature-conditioned air into a rear passenger zone through at least one zone outlet, said housing assembly configured to direct system inlet air through said evaporator and to trifurcate said inlet air into a first cold air stream directed into a first mixing chamber disposed in said upper mode case, a second cold air stream directed into a second mixing chamber disposed in said lower mode case, and a third cold air stream directed through said heater core; an upper temperature blend door operable to proportionally control the ratio of a first hot air stream exiting said heater core with said first cold air stream as they enter said first mixing chamber; a lower temperature blend door operable to proportionally control the ratio of a second hot air stream exiting said heater core with said second cold air stream as they enter said second mixing chamber; an upper zone mode valve operable to selectively obstruct said discharge of said first stream of temperature-conditioned air into said front passenger zone; a lower zone mode valve operable to selectively obstruct said discharge of said second stream of temperature-conditioned air into said rear passenger zone;