No air from vents with fan on

[J.B. Hebert]

Recently, I noticed that I was getting little or no air from any of the 
vents (dash, defrost, floor) on my '91 Coupe, even when the fan on 
high.  I checked the intake vent for blockage and pulled the fan out to 
inspect the heater core to see if something was plugging it, but 
everything is clear.  I also checked the function of the fresh air flap, 
and it seems fine.  The fan spins fine and there is no damage to the fan 
cage.  This problem seemed to have occurred suddenly, so I'm wondering 
if one of the diverters is stuck or blocked.  Has anyone run into a 
similar issue, and if so can you point me to some possible places to 
check before I pull the entire dash?  With temps in the high 90's 
lately, I'd really like some more air flow.

Thanks in advance.

J.B. Hebert