Q: How to remove and install a heater core in a Mercury Mariner?
A: Disconnect the cable from the negative battery terminal. Drain the cooling system, then disconnect the heater hoses from the heater core inlet and outlet pipes at the firewall. Squeeze the tabs on the spring clamps together with a pair of pliers and slide them up the hoses to disconnect the heater hoses from the heater core pipes. Remove the instrument panel. Carefully pry the lever off the blend door shaft. Remove the actuating rod from the lever. Remove the heater core cover screws and detach the cover from the HVAC housing. Pull the heater core out of the housing. Installation is the reverse of removal. Don't forget to reconnect the heater core inlet and outlet hoses at the firewall. Refill the cooling system. Have the air conditioning system discharged and recovered by an approved service facility. Disconnect the cable from the negative battery terminal. Drain the engine coolant. Remove the entire instrument panel. Disconnect the air conditioning refrigerant lines from the engine side of the firewall. Disconnect the heater hoses from the heater core at the engine side of the firewall. Remove the heater core housing mounting nuts from the instrument panel studs. Pull the right side of the heater core housing to the rear, then slide it to the right to release it. Remove it from the vehicle. Remove the seal from the heater core tubes. Remove the bracket that secures the heater core, then slide the heater core out. Installation is the reverse of removal. Check for coolant leaks, then have the air conditioning system evacuated and recharged by the shop that discharged it.