Yup, I check the blend door actuator, I watch it work, the dial works the inside air/outside air door on the passengers side low down. The vacuum door behind the radio operates open/shut....there's great vacuum to these two units....I replace under the hood vacuum line with new......
so, where's the actuator that's NOT closing the defroster vent to vent air????.

I'm going nuts trying to find a diagram of this stuff!!

Behind the glove box pictures....

 

Problem sounds like the defrost valve is broken.
Above the accelerator pedal and to the right are two actuators, one on top of the other. One of these controls the defrost valve, I think it is the upper one. Anyways if you remove this actuator and twist the shaft sticking out from the air box you might be able to hear the defrost valve flopping around, most likely the shaft has broken and you are not turning anything.
Some have pushed a stiff wire down the defrost vent and wedged it closed so that AC blows out the dash vents. Which is fine until you need defrost.
The defrost valve is inside the air box. Some have cut a hole in the air box to change it, then sealed it again.
The only other way is to remove the dash.

Try doing a search for defrost valve....something might show up.

have fun with this one....

 

Yes! This may be it, I removed more screws/ bolts on the drivers side this morning....found that dashpots/actuator you speak of....and when I cycle thru all the choices, all the doors work, except this things rod never moves....and it's hard to get to.
im not sure how to get to it, or how the dash come out....time to search the web, and stop at the local Chevy dealer....
they may be open Monday. Surely there's a way to fix it.....if I have to disassemble the jimmy to get to it....you can be sure I'll go 1970's....and put a manual cable on it....like I did the 4 wheel drive....stupid engineers....electric to vacuum to mechanical.....😖

thank you you for your help.....I'll update the sad saga...Marc

 

The chief mechanic at Chevy dealer just told me, most likely I'll need to continue and pull the dash. This allows access to this mode valve...it needs to work WITH the upper to direct the airflow correctly. Part is no longer available from GM, but doorman makes a close copy...
stay tuned! The saga continues 😖

 

Here is a picture of what I am referring to, The lower actuator is the one that controls the defrost valve. The white shaft coming out of the air box is the defrost valve...which is really just a flap. I have a new one in a box if you want a picture of it.

 

Oh and taking the dash out is not all that difficult, just required patience and time to remove all the screws/bolts/electrical connectors, drop the steering column down (support it) and have another person to help lift the dash out.
Probably most difficult part is remembering what screws/bolts go where!
And be careful with all that 20+ yr old plastic...especially the electrical connectors.

 

WAIT!!!....just noticed that you have a 1st Gen Blazer....everything I wrote is for a 2nd Gen....doubt that any of it applies.

 

Sorry to hi-jack...Lanny I'd love to see that pic of the new flap you have. Mine's loose at the white shaft and I need to replace. Everything I'm finding online is not clear if the shaft and the flap are one piece or two.

 

Took the dash out completely....and it's heavy!!!...anyways tested the one actuator, it is working. It may be the 'switch' that uses the vacuum, but the part is expensive......so I put a balsa block and MADE it push out the ac vents😡

I got the dash back in, but will need a few mornings to reassemble the jig saw puzzle back together.....

No easy way to do this....but at least i understand the system now...

 

This is what I have for my '02: