1983 Porsche 928 Heater HVAC vacuum testing

1983 euro Porsche 928 Hvac testing! A couple of questions if anyone can help? What feeds the heater control valve solenoid? The heater controller or the setting motor? Why does my white vac line going from the solenoid to the heater control valve tee off and have a separate white line going to somewhere behind the glove box? Any advice appreciated! :) #Porsche928 #928 #ProjectCars #928S #V8