Steve, I would like you to check for available vacuum for the HVAC. You may have a broken plastic line, or leaking check valve. If the vacuum under the hood is OK, then pull out the HVAC controls and check for available vacuum there. Here is why: The temperature blend door you are describing is positioned by an electric actuator. And, they certainly have their issues.. However the airflow mode doors are positioned by vacuum actuators. In the absence of vacuum, all resulting air flow defaults to defrost. So check the vacuum first. Then we can talk about whether pulling the dash is even necessary. Please get back to us!