Now here is a task everyone loves. This procedure, for a non air car in this case, is recommended even if just the heater core is to be replaced. It makes getting to the heater core easier and you won't be trying to pull the metal dash rearward and bending it out of shape. The heater box is mounted by 5 nuts accessed at the firewall under the hood and a bracket where the cowl vent meets to the box at the far right. Many heater boxes have been broken because the fifth nut is overlooked, and the case is broken by attempting to force it out of the firewall. That fifth nut is just below the vacuum tree at the center of the firewall under the hood. The heater box case comes apart by prying loose the spring clips positioned around the edge of the case front cover.
1. Remove the lower metal dash as outlined here. In many cases, it may be enough to loosen the lower dash just enough to move it out of the way for you to get to the heater box. This is a decision to be made at the time.
2. Drain the radiator.
3. Remove the heater hose clamps and hoses at the engine and lay the heater hoses loosely over the engine.
4. Remove the five nuts under the hood.
5. Remove the bolt holding the box inside the car at the cowl vent and unplug the blower switch connector on the case.
6. Remove the screws holding the adjusting cables and remove the cables from the box if this has not been done.
6. Move the passenger seat back as far as possible, sit down in the floorboards, and grasp the heater box at each end.
7. Move the box down and toward you, slowly pulling the heater hoses through the holes in the firewall.

Now you have access to the heater hose clamps at the heater core. From this point, the box is ready to be removed from the car. It is recommended that you consider replacing the core, seals and gaskets inside the box, as well as heater hoses and clamps.