I can think of two reasons for cutting a vent into a door, temperature, or fresh air. Cut the vent in the upper door to vent heat or natural gas, in the lower door to vent cold or petroleum based gases, LPG, diesel fumes, ...
No, you can not alter the door in any way and maintain it's fire rating.
should be the same door and yes you should be able to eliminate the vent window as it just bolts in the door, I didn't think a 68 had a vent window.
Driver side below the dash and right next to vent it has a hole in it that ties to a shaft inside the vent i think there is two screws that hold it in place.
vent door will be open to what ever setting u set it on regardless of what temperature setting you are using
The vent door actuator is operated from the engine vacuum. If the vent control doesn't respond to your demand check the vacuum supply line from the engine, it may be damaged or disconnected. Do this before you tear apart the dash and HVAC box.
It's called the mode door actuator and it is up in the vent system.
It controls which vent the air comes out of.
It controls which vent the air comes out of.
take inside door panel off and the vent window is bolted inside the door as it also I believe acts as the front window channel.
The air is all heated in the same place. If you have floor heat and no defrosters then the problem is likely in the vent doors or the vent door controller. If you have no heat at all then it's in the controls or operation of the heater core.
IF you have a woodburning fireplace, and there is a small door- about 6 inches square on the outside of the house, check for a flip open door on the floor of your fireplace. If that is what you have, this is not a vent, but an ash clean-out. Shovel ashes thru the door on the floor of the fireplace, and you can remove them from the outside door without making a mess in the house. But not a vent. If that is not what you have, please post gain with a more detailed description.
The heater door control on a Blazer can be replaced by purchasing a new one. The heater vent will pop out with a little nudge from a flat tipped screw driver. Then insert the new heater vent door control.