Heater problem

Hello:
 
     I have a 1999 Expedition which I love dearly.  Best vehicle I ever owned.  However, the heater has just started acting funny.  The front heater will not blow hot air, no matter how I set the temperature.  The A/C works fine, but the heater in front blows cold air.  Defroster is cold only too.  The rear heater works fine.  No problem.  I purchased a shop manual, but can't for the life of me figure out what's wrong.  How can one heater work and not the other ?
 
     I read somewhere that it might be a problem with my "Blend Door".  What the hell is a blend door and where on the vehicle can I find it ?
 

The blend door is in the Heater A/C plenum. It's  function is a 'flow gate' for heat and A/C temp control.. It's driven with a blend door motor..

Here's a couple of images of the blend door.. One in the 'OFF' position and the other showing HEAT/ Blower 'flow' through the heater core.. 
 

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Great info.  I see how it works.  Where exactly is it located though.  Is it something I can get to and try to work on, or is this a dealership issue ?

The Plenum Chamber is located 'behind' the intrument panel, and behind the glove box area on over to about the center of the dash.. If you have a no heat issue it should be diagnosed as to the cause prior to any dash work if possible.. If a bad blend door, or door actuator/motor is the cause, then it's a 'sizeable' job to complete..