vacuum for the HVAC starts in the area around the EGR solenoid, comes off one of the larger pair of black hoses top of manifold. ( has mulitple uses) Goes down along the firewall. Now under the car find the heater hoses center and you will see a single vacuum line. This is the supply to the HVAC.
I've never had mine quit--yet--but this question has come up many times here. Of course the leak or break seems happen in the least accessible area
Lost of vacuum the system deflauts to only defrost.
Paul
