The Expedition uses the same heater box as the F150 and Lincoln Navigator. The blend door acutator motor is located on the bottom of the box and the blend door turns on a vertical axis. The most common failure for these cars is for the plastic connection on the blend door to motor to break. It is very common to remove the blend door motor and have a handful of black plastic pieces fall out. This is a sure diagnostic of a broken blend door.
The motor on the Expedition is located above the transmission tunnel. To get to the motor you will have to remove the center console. You will see a black plastic vent under the box. This is for air distribution to the passenger and drivers side floor boards. The acutator is just above this vent and the vent has to be removed to get to it. The Expedition was not designed for repairing this problem and the Dealer process is to remove the steering wheel, steering collumn, complete dash panel, evacuate the AC, drain the radiator, and finally remove the heater box to get to the actuator and blend door.
For us cheap guys, the bottom vent can be removed with enough force and creativity. You may have to cut it with a dremel tool to remove it. This is not a structural or high pressure vent, so it can easily be repaired with metal duct tape and is completely hidden when you replace it. You can also reconnect the motor and see if it is working properly. Once you get the the motor, it is easy to remove the screws and pull it straight down. Look for broken pieces and/or examine the connector with a mirror for signs of cracks. You can manually turn the blend door with a flat piece of bent metal and switch from AC to heat to see if the door is working properly or hanging.
If the door is broken, you have two options. Take it to the dealer and have them go through the process above and replace the door with another plastic door that will also break over time, or check the HeaterTreater fix on Ebay. This replaces the door with metal and the fix is done through the glove box opening without the 12hrs labor to remove the dash.