I am removing all my AC components on my drift car and I have pulled the dash to get to the boxes behind it. Which of the three pieces back there would I need to keep in order to have working window defogger?

 

you need a/c to have a functional defogger. course the heater works but takes much longer and requires it to be on full all the time to work even half assed.

 

And if you just wanted to blow air on your windshield aimlessly, then you have to leave the blower and the heater unit. The evaporator unit, which is the middle one could be removed. You would then need to build some sort of duct to get air from the blower to the heater unit, which would still have the air mix motor attached to it to force the air up the front dash vents.

Demisters/defoggers often work best with cool air. Read why:

http://en.wikipedia.org/wiki/Defogger