The Mercury Grand Marquis is one of many vehicles in which the heater unit is located behind the instrument panel on the passenger side. Replacing the heater core in the vehicle means removing the heater plenum. You will need to open up the instrument panel in the car to do this, along with disconnecting the core on the engine's side. You will likely need to change the core if your system is leaking coolant into the car.
Disconnect the negative battery cable and drain the engine coolant -- connect a hose to the drain fitting and open the fitting to drain the coolant into a container. Disconnect and plug the heater tubes at the engine's firewall.
Unscrew and remove the three nuts right below the windshield wiper motor and the single nut at the evaporator case's left side corner using your wrench. Disconnect the vacuum hoses from the vacuum source and push the hose leading through the firewall into the passenger compartment.
Remove the instrument panel trim moldings and insulators and then the steering column covers. Disconnect the gear shift cable, remove the brace under the column, remove the column's retaining nuts and lower the column.
Open and lower the glove compartment door and remove the instrument panel's retaining bolts on the left side of the opening. Remove the sill plates and cowl panels at both doors and defroster grilles and remove all the concealed bolts.
Pull the dash as far as you can without disconnecting any wires, then remove the cross-body brace and disconnect the temperature blend door's wiring and the temperature sensor tube from the evaporator case.
Disconnect the vacuum jumper harness at the floor air duct and the vacuum hose from the recirculating vacuum motor. Remove the floor duct by removing the push fastener and the left-side screw, and by loosening the right-side screw at the plenum's rear end.
Remove the plenum's two nuts along the lower edge, then move the plenum towards you until its heater core tubes clear the holes in the dash panel. Rotate the top of the plenum forward, down and out from the dash.
Unscrew and remove the heater core cover and pull the core and its seal out of the plenum.
Insert the replacement core within the plenum, making sure its seal is correctly seated around the core. Close up the heater core cover with the screws.
Route the car's vacuum supply hose through the dash panel and seat its grommet in the opening.
Insert the plenum under the instrument panel with its heater core tubes down and the register duct opening up, then rotate the plenum upward, position it to the dash panel and insert the tubes and studs within the dash panel and evaporator case.
Fasten the plenum and the floor air duct in place with their retaining nuts and pins. Reconnect the vacuum hose and jumper harness, the temperature sensor tube, the blend door wiring harness and the cross-body brace.
Push the instrument panel back into place and apply all its bolts. Reconnect all the trim panels on the dash and raise and fasten the steering column.
Reconnect the vacuum supply tubes to the vacuum source, the nuts on the plenum's studs and the heater hoses to the firewall fittings. Reconnect the negative battery cable.
Fill the radiator with a fresh mix of engine coolant and water, unless you can confirm that the old coolant mix is perfectly clean.
