Heat in front blows cold, heat in rear blows warm. why?
Heater in front blows cold on 2002 Ford Expedition
Most commom problem is blend door problems in the front HVAC unit. Will need to have someone familar with those type of repairs to check and see if what your problem is.
Some Ford trucks have a vacuum operated valve in the heater line for the front unit. Check lines to and from unit by feeling if they are hot. If not valve may be bad. If lines are hot then it could be a Blendoor or possibly a control problem.