1999 Ranger 4 X 4 Heat / AC / Air only blows through floor and dashtop

My 1999 Ranger has 78000 miles and the heat / ac only blows through the floor and defrost vents at the same time. It will not blow throught vents on front of the dash. What could be the problem ?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
vacuum leak, it re-sets to default

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.