My heat heat won't go to vent or floor, blows strong out of defrost vents but does not switch to other vents.

SOURCE: There is no air flow

You need part GM Part number 89018675 or ACDelco 15-73596. The Mode Selector Actuator. What happens is the part is a CHEAP part POORLY designed and when it goes to turn the flaps inside the HVAC unit its the plastic gear slips instead of turning the flap!!!

Just to forward you you might want a deal to install this art for you unless your good at fixing your own vehicle, because you need SMALL sockets and a lot of patience to get this part in!!!

The part is ALMOST directly above the steering shaft for your steering wheel, so theres a lot you got to remove before you will even see them!!!

I only recommend you to have a dealer put this part in for you, unless you have your own service manual!!

Because there is 5 of these darn things and if you pick the wrong one you will have to do it all over again!!!

3 on the Drivers side and 2 on the Passengers side!!

But replacing this part SHOULD solve all your problems!! AS it did for me and i was having the same issues as you have described!!

SOURCE: heater control 98 chevy s10 blazer

These trucks have a problem where moisture builds up in the heater box then freezes. When this freezes and you change your heater settings it breaks the shaft for the blend door. If this is what happened you should be able to look under the dash above and to the right of the gas pedal and you will see two vaccuum accuators one of them you should be able to wiggle around the linkage if it is broke. To fix this you need to remove the entire dash to get the heater box out which involves discharging the AC and draining the coolant because the heater core and evaporator come out. Please rate.

SOURCE: A/C only blows through defrost vents

It sounds like the blend door actuator which diverts air from defrost to vent to the floor.
The bad news is it's a bear to get at. It's located behind the glove box.

SOURCE: 1996 chevy blazer my air on heater will not blow

the idea of the switch is run on vacuum and is not making contact with the switch it self either worn contacts in the switch itself **** as an obstruction as well or the vent door is stuck in just that position either behind the glove box or the ash tray are the vacuum lines one of them may be pinched off or the door needs a bit of help until you can free it up

SOURCE: All Air Conditioning blows our the defrost vents

Some thing you could look at would be this. The control is usually fed by a vacuum line or small hose thats is connected to your intake manifold.
With a cold engine,look for a skinny vacuum hose that runs from the intake toward the fire wall . It should be going inside the cab and connecting to the A/C vent control.Some times the hose becomes disconnected or splits or cracks due to heat under the hood.
Thats usually the trouble.
I hope this helps you.