The prediction of water ingestion by the air conditioning system is an issue affecting either the cockpit comfort or the reliability of an automobile. The purpose of the HVAC airbox is to provide a clean and dry airflow to the conditioning system. However, the presence of water in the airbox makes this task complex. Among the main causes, infiltration through the gap between the wiper shaft and its bushing was noticed during validation tests. this infiltration is governed by the pressure difference between the airbox and the outside environment, which are present when the HVAC is activated. An analytical model is proposed, which, coupled with a numerical simulation of the flow within the airbox, is capable of predicting the conditions causing the infiltration.