Does anyone here no how to fix the little door in the top of the heater core so that it will open when you turn on the heater, hence letting hot air coming out of the dashboard vents, instead of only the defrost vents?

Travis

 

Is the door itself broken or is it not opening? My first guess is that a vacuum line has either broken or come off, since the door is operated by vacuum. Check under the dash and also in the engine compartment for vacuum lines that are broken or disconnected.

You should be able to hear the door opening and closing when you turn the dial from floor to defrost. If not, start looking at the lines. If you can't find one by visible inspection, you may need to turn the engine on and listen for a hissing sound from a broken line.