It's stuck on Defrost /floor at the moment.
I think it's a vaccum accum issue.
I haven't found any leaks under the hood, 4x4 works.
There is a hissing coming from the dash behind the selector switch.
Is It normal for some, not much to leak from the back of the switch that has the 5 colored lines going into it?
That switch is the only place I can find any leaking.

 

Should not have any hissing or leaks there. That is your problem.

 

Thanks a ton!
Do you know what that switch is called?
96 blazer hvac switch isn't bringing up the right part on google

 

Believe the default is defrost if no vacuum.
Attached is 1996 HVAC manual with vacuum schematics.
Plastic lines and rubber fittings are 25 years old so take it easy.There is a plastic vacuum tank INSIDE left fender across from master brake cylinder. Check the hoses as there is a T for 4WD and HVAC

 

Holds at 20 on the vaccum gauge wile it's running drops to zero as soon as the key is turned off.
Found and fixed a leak near the transfer case, the switch on the case was also froze up.
I replaced the switch.
Now it changes from all floor, all windshield, or blend floor/ windshield, but the vents are still a no go
I made a smoke tester and cant find any more leaks or collapsed lines.
Is there a different actuator that moves it from window and floor to vents?
I found a actuator looking thing with a orange vaccum line coming out of it. But it looks like it is to let outside air in

 

A clever way to diagnose a vacuum leak and start isolating which line its coming from is to watch fuel trims while the truck is running at idle and your start pinching off lines and branches until the trims go closer to zero.

George

 

Have you checked the engine compartment vacuum lines to see if they are leaking?

If you are losing vacuum and your HVAC actuators are not working properly, you likely have a leak to thr vacuum reservoir.

 

The lines in the engine bay are old, look like possibly factory but not cracked.
I'm going to go to autozone tomorrow and get enough line and fittings to replace all but the little hvac lines just to be safe.
Going to pull the actuator and check it for tears as well.
Theres a faint hiss coming from the hvac panel, and when I smoked the system it was smoking from the Grey line, and a line on the transfer case.
Fixed the tcase line, but couldn't get the one on the hvac panel to seal
Wile it's running its holding between 17-20 on the vaccum gauge, but drops to zero as soon the key is turned off.
Because it's holding decently wile it's running and since I fixed the break by the tcase, I can now put it on only floor, only defrost or a blend of the two instead of it being stuck on only a blend of defrost and floor I'm thinking it's a bad actuator under the dash somewhere.
I'll find out for. Sure after I replace all the lines tomorrow.

 

Oh you applied smoke to track down the leak? Nice.

And yes, you might as well replace those under-hood hoses. If they have not cracked yet, they soon will. I used my drill bit set to work out what the ID of each was. I had to cut the lines in the middle where they were not stretched to get a good measurement through.

Good luck!

 

Yeah I picked up enough stuff at Lowe's and thin wire off Amazon and made a home made smoker.
Put some baby oil inside, and close the lid, hook the gator clips to the battery, hook it to my compressor, tie it to the resivor vacuum line.
The current cooks the baby oil to a smoke, the compressor pushes it through the vaccum system.
Cheap and worked very well, may have 30 bucks in it