69 Barracuda heater control vacuum lines on firewall

Defrost may not be default in this particular case. Reason being, majority of actuators/ servos used are a single line vacuum over spring type.
Chrysler engineers used several dual vacuum (no spring ) servos. When vacuum is lost, these could stay in last called for position unless another force causes movement.
Dual vacuum servo at hot water valve is one example. Take away its vacuum and water flow could push the valve open.