I can't switch from air to heater. It's like something is blocking it.

Hi Brad, Did your windshield Fog up immediately when you switched to defrost? Can you also feel air coming from the vents at the top in defrost mode? If so, Could be your heater core is leaking.....If not I'm going to need some more information. Hope this helps, Good Luck, Randy.

I’m happy to assist further over the phone at https://www.6ya.com/expert/randall_d932ef6e01588b0f

Could be the heater core; on a Pinto I had years ago, that was abut the heater core location. Does the water smell of antifreeze?

there is a paddle inside the sysem which moves to change the direction of the heat, i could be jammed or its actuator is broken or not getting pwer. if not it could be your switch is not working so the sigal to change position is not be received. first check all fuses are ok.

I hope this was useful.

There is a blend door actuator broken its what switches the air flow from floor to vent to defroster. It's a real pain in the *** to replace under dash and depending on what one is broken glove box or maybe center console must be removed

this could be many different issues. you could have a lower degree thermostat, if the block wasn't boiled and flushed you could have coolant restriction in the block causing less flow to the heater core,or you could have put your heater core hoses in reverse flow.

That is obvious
The job wasn't done correctly

Most likely wasn't sanded down enough &
properly sealed,primed Etc depending on the
repair required

Check your coolant level, if its low it may indicate a heater core leak into the vehicle interior. It will severely fog up the windshield and seem like an oily deposit on the glass. If good just keep the heater on defrost to the windshield or blend to defrost and heater.

Under your hood, passenger side bolted right in the middle of your air conditioning unit is a lone relay screwed in with 2 screws. That was the reason my 92 Blazer wouldn't blow 99% of the time. Knew it wasn't likely the motor since mine worked sometimes, and motors usual are just good or bad. The low/med/high fan speed switch has a resistor in it that can also go bad, I changed that 1st, no change, swapped out that $6 relay and now I have a fan again :) Those are really the only 2 things causing your problem unless your wiring is a wreck or something unusual. Bonus: relay's a 5 min fix taking your time.

OUCH ITS THE SPEED CONTROL, THE MOTORS MUST BE IN GOOD CONDITION ,HAVE THE AMPS CHECKED ON THE MOTOR NEEDS TO BE UNDER 20 AMPS! ON THIS TYPE OF SYSTEM THE SPEED CONTROL IS UNDER THE HOOD ABOUT CENTER NEXT TO THE WINDSHIELD HAS BIG WIRES HOOKED TO IT AND IS SCREWED INTO TO THE DUCTING, IT HAS A BLOCK OF ALUMINUM WITH FINS THAT COOL IT THAT CAN BE SEEN WHEN REMOVED. GO TO PART STORE ASK THEM FOR THIS PART SO YOU CAN LOOK AT IT IF YOU CAN'T FIND IT UNDER THE HOOD. THIS PART YEARS AGO WAS OVER 300.00 NOW ? ITS BEEN A FEW YEARS I THINK THERE DOWN TO 60.00 OH AND ONE MORE THING MAKE SURE THAT THE MOTOR HAS A GOOD GROUND.. SOME CADS HAD A GENERIC SYSTEM AND IT COULD BE JUST THE FAN SPEED CONTROL BALIST.. THERE COOL CUZ THERE DIRT CHEAP THREE COILS OF WIRE. LOOKS LIKE SPRINGS

If you can feel the air from the blower motor blowing up on the windshield it is probably one of two things. Either your heater core has a leak and the anti-freeze mist film is all over the windows or your heater control valve is not working.
Does the heater get hot or can you feel hot air on the windshield? If so your heater core is probably leaking and making steam inside your car.