I had the same problem on a 97 STS. The system on my car works with 2 small rubber vacuum hoses connected near the air conditioner receiver/dryer to hard plastic tubes. The rubber vacuum hoses had dry rotted and were leaking. The 2 short rubber vacuum hoses go near the firewall and connect to a small diameter hard plastic tube that runs through the firewall to the control unit inside the dash... I replaced the 2 short rubber vacuum hoses and the air flows correctly.