The Toyota Highlander uses two heaters whose cores may need replacing. The rear heater is slightly easier to get to, located near and below the rear console, but still requires removal of some components. The front heater is much more challenging, requiring removal of the whole instrument panel.
Disconnect the Highlander's negative battery cable. Drain the engine coolant into a clean container if you want to reuse it. Disconnect the heater hoses from the rear heater core.
Take out the front seats, front, rear and lower door scuff plates and the cowl side trim. Remove the rear console box and the left side air outlet grille.
Pull the carpet toward the rear. Remove the air outlet grille and rear air duct and disconnect the electrical connectors.
Remove the rear heater housing and the heater core-to-heater housing screw and plate (by disconnecting the screws and clamps). Remove the heater core from the housing.
Install the replacement heater core into the housing and connect the housing screw and plate. Reinstall the rear heater housing.
Reconnect and install the electrical connectors, rear air duct, air outlet grilles, rear console box, door scuff plates, cowl side trim and the front seats. Connect the heater hoses back onto the rear heater core.
Refill the cooling system, using fresh coolant if the old coolant does not appear good. Reconnect the negative battery cable.
Unplug the negative battery cable, drain the coolant and disconnect the heater hoses from the core.
Detach the steering wheel. With the front tires facing straight ahead, remove the wheel side covers and screws at each side. Remove the air bag module and unplug the electrical connector, then remove the wheel nut. Place alignment marks on the wheel and main shaft.
Pull out the instrument panel after disconnecting all components on the panel. This includes the front door interior, steering column, hood lever, glove compartment, center console, clock, radio and heater controls and the audio amplifier.
Discharge and recover the air conditioner refrigerant. Remove the evaporator housing after disconnecting the refrigerant lines and electrical connector. Remove the defroster nozzle, theft deterrent, wireless door lock ECUs and the air duct.
Remove the heater housing from the chassis, the cover from the housing, then remove and replace the heater core.
Reinstall everything in reverse order of removal (heater housing, defroster nozzle, evaporator housing, instrument panel, steering wheel, heater hoses). Fill the cooling system and reconnect the battery cable.
Evacuate and charge the air conditioner. Run the Highlander's engine until it reaches normal operating temperatures. Watch the climate control operation and check for leaks in the system.
