Why is there no air blowing through vents in front?

Possible vacuum leak under the hood at firewall. small hard plastic vacuum tubing that goes through the firewall may be split/cracked/acid corrosion if they still do the newer Expeditions like they use to. common problem on the older models for this,but most of the time the air would default to the defrost mode? also copy and paste the link below,not sure about it. https://repairpal.com/ac-fan-speed-changing-constantly-793