96 chevy blazer air cond.

check and test with a meter the fan --fan speed control module [ attached to fan housing ] --switch and relays ---

Can be blower motor easy to test --remove it look for imbalance by spinning to check bearing smoothness / freeplay . Hook up ground then use inline fuse connector with lower than fuse panel ## number of fuse -- find power Eg .car battery -- be sure to ground then connect Fan to visually see it spin .. NOW if you have any type of electronic dash mounted controls for heat / air conditioning - most likely electrical issue is in it . you said blower motor does sometimes function - Test wires that plug in to it for power / find groung then probe by test light when in heat mode = switch to air cond mode if fan functions then problem in controls on dash .. if no test light reaction search wires for short or damage .. As FUNNY as it sounds I have pulled mouse nest out of GM heater fans - They access parked vehicle by drain hole inner fender well it drains water from wiper blade / heater vent area .

Have you checked the rear blower motor for power and ground ?

Faulty blower motor resistor or blower motor resistor connector faulty.Wiggle blower motor resistor connector with the ignition and fan switches on.

without being able to trouble shoot the switch i don't have a solid answer for you but I would guess that the resistor in the switch burned out and a new fan switch should take care of the problem. good luch

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

no most likely air vents are clogged

Your heater valve is stuck open, also in a pinch you can crimp off your hose to the heater coil at fire wall. Try to find the heater valve vacumb hose under the hood see if it is bad and fixit. or some models have a cable to the valve could be broken. 

Is it your ventilation not switching, or your blower motor not blowing air?

i would say its in the dash unfortunately you will have to pull it out to inspect.