blend door on xterra 2003

I have Xterra 2003 (3.3L not supercharged), and the heater is not working any longer. The blower motor is working properly, blowing air through the vents, but whatever the temperature setting, the air is always cold. I read somewhere that it may occur if the blend door is stuck in some position. Does anyone know where that blend door is and how to fix it?