Defrost help

[Dr.Destructo]

Hi;

I need to get the defrost working on my '83 Blazer so that it will pass safety certification up here in Canada.

The heater and heater motor works fine, and the lower vents blow hot air. When the selector is set to "Defrost" (the vents on the upper dash near the windshield), no air blows through them.

I thought I would ask here to see if anyone had a similar problem.

I am planning to take the upper dash out of the truck to see if I can investigate the problem. Could one of the ducts have been kicked off? Or maybe it is plugged up? Is this fixable by a diy-er?

A mechanic friend said it may have something to do with the A/C on the truck having dust accumulation?

Any ideas would be greatly appreciated.

I have the service manual for the truck but it doesn't go into the venting/heating system.

Thanks.