A modular door for a vehicle HVAC system includes a door plate, a first shaft, and a second shaft. The door plate has a front surface extending between an upper edge surface and a lower edge surface. The upper edge surface defines a first array of keyed orifices. The lower edge surface defines a second array of keyed orifices. The first shaft has a first keyed protrusion that is disposed within a first keyed orifice of the first array of keyed orifices. The second shaft has a second keyed protrusion that is disposed within a first keyed orifice of the second array of keyed orifices. The first keyed orifice of the first array of keyed orifices and the first keyed orifice of the second array of keyed orifices form a pair of axially aligned keyed orifices.