The 1999 Ford Expedition features a heater core used for heating engine coolant and distributing it via a fan to the cabin for heat. If the heater has stopped working, it's likely the heater core needs replacing. While you can save a great deal of money by replacing the heater core yourself, you should hire a professional to discharge your air conditioning system before doing so. The refrigerant used in the air conditioning system is highly pressurized and potentially dangerous if you are not experienced enough to deal with it safely.
Unbolt the negative battery cable with a ratchet and socket, place a drain pan beneath the radiator drain plug, unbolt the drain plug with a ratchet and socket, and drain the radiator.
Lock the steering wheel and carefully pry off the trim, center console and instrument and kick panels with a small plastic-coated pry bar. Pull the green grounding cable and electrical connector tabs gently apart with your hands, lift the passenger air bag out, unscrew the defroster grille and mounting panel using a flathead screwdriver, then lift the mounting panel out with the help of a friend.
Press the tabs together on the heater hose and evaporator lines, pull them out, then plug each line with a rubber plug.
Pull the vacuum line out, remove the heater core mounting bracket and the heater core with a Phillips screwdriver.
Install the new heater core, and follow the previous instructions in reverse to complete the installation.
