heater woriking but not getting air through vents. Hear blower on. on 1999 Ford F-250 Super Duty

My heater is working. I can feel hot air. I can adjust the blower and hear it working, but air is not coming through vents no matter what the setting. I think it is a vacuum issue but cannot narrow it down.

Asked by for the 1999 Ford F-250 Super Duty
It is definitely a vacuum issue. Most likely a hose disconnected under the dash or under the hood. Look under the dash - both sides - first.