How Much Air through Defrost Vents

[Zilverado]

When I drove my truck a few months ago, for the first time after parking it for the winter, it seemed as if there was not very much air coming through the defrost vents. It was a warm, rainy night and the windows fogged up pretty good - even with the fan on high.

With the air is set on defrost, there is still a fair amount of air coming through the a/c vents and the floor outlet (not as much as the "heat" setting, but still quite a bit).

I haven't had too much difficulty with it since (haven't driven in the rain I suppose). But I thought I should look into it before winter comes. So I tore into my dash to see if I could find anything obvious (i.e. loose vaccuum hose, etc.), but all seemed normal.

So, how much air is normal? Should I not expect most of the air to come out of the top in "defrost" mode?

Thanks,