92 burb defroster

I got a 92 suburban and the defroster don't work correctly, the motor that moves the deflector is working but no air comes out the defroster vent, I get a high pitched whistle so I think maybe something has fallen down in
there. Has anyone tore into the duct work under the dash or had this problem before? I guess my question is, how and where do I start to take this apart and find the problem?
Thanks, Tim

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

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.