Plenum assembly in vehicles are used for the primary function of air inlet to the HVAC during fresh air mode operation. The secondary functions are water drainage, aesthetics cover for the gap between windshield to bonnet, hide wiper motors and mechanisms etc. Currently majority of global OEM designs employs a plastic upper plenum and a lower plenum made of either plastic or sheet metal which spans the size of cross car width of engine compartment. In this paper a new geometry shape is derived for the plenum lower assembly which is developed and further fine tuned using vehicle testing. The construction uses all plastic material with full serviceability of plenum design. This enables the design the potential as a standalone unit which can be cross deployed in similar class of vehicles instead of going for new development, thus enabling significant cost savings. The proposed lower plenum geometry consumes about 1/3rd of the packaging volume compared to a conventional plenum in similar class of vehicles, and with a weight savings of up to 50% of conventional design. The paper also describes on the integrated plenum upper drainage systems of plenum upper, which is developed for this new geometry of the lower plenum. A optimised and structured approach to testing of the plenum is derived using the worst case rainfall data to correlate to the real world conditions. The proposed new plenum design observed to perform better in all test parameters and was implemented on TUV3OO.