you have an actuator that is just above the center console under the dash- This will have one of the colored lines that pulls vacuum on it- It is to the driver side of the center console- That controls air movement And you can override it with your hand - If the actuator is not holding vacuum then it needs to be replaced and they are sold at parts stores-
There is another one that is basically behind the glove box and to the right behind the passenger side front speaker-
You can literally crank the vehicle and turn the selector and see these do their job
The blend door is electronic and variable , it is basically wired in above your left shin if you are in the passenger seat- You can put on a stethoscope and hear it operate
I would start with the one that’s just about the console- Don’t be intimidated by this it is a very simple system-
It is also possible to lose vacuum at the switch in the dash- or the line from the switch to the actuator- but the actuator is can stick, become unhooked or leak.
P.S. - I’m not trying to take over the conversation I just have this fresh in my mind and wanted to offer what I could- dont let it intimidate you - there are only 3 zones (defrost, dash and feet) controlled by 2 actuators if I’m correct, all the switch does it distribute the vacuum pull, and the primary blend door is electronic - im sure there is an actuator for the fresh air vent to , but it likely is not causing the zone issue- Andy