There's no air coming through through the front defroster vent, leaving me with a very foggy windshield.  Blower works and plenty of air from the mid level and floor vents .

 

I took the blower out and there's nothing I could see blocking the air passages in the box above it. 

 

Any ideas how to investigate this ?

What model car?

I keep the model information in my signature

 

The good news is I found the problem.  A vacuum line from the intake manifold was disconnected.  Reconnected it and all is well.  Wish I had found that before I removed the blower.

 

Thanks for replying.

Oh, I didn't see it.  It is in grey on white, hard to read.  I had a 4WD wagon with all the same as yours.  Until the frame broke from rust.  I would have suggested the vacuum line or lines after the selector buttons.

I keep the model information in my signature

 

The good news is I found the problem.  A vacuum line from the intake manifold was disconnected.  Reconnected it and all is well.  Wish I had found that before I removed the blower.

 

Thanks for replying.

 

Yeah the first time that happened to me was on a cold evening. Also, the vacuum storage tank can sometimes leak. I had to replace the one on my Loyale 5 years ago. I think they said it was the last one when I ordered it from Subaru.