Installing a heater core in a Chevy Cavalier refers to replacing the part of the car's heating system which controls the temperature. Although it is a fairly involved process, it is not too complicated, so anyone with a little automotive know-how can accomplish this repair on their own. Follow these steps for how to install a heater core in a Chevy Cavalier.
Detach the negative battery line.  X Research source
Lift the front end of your car and prop it up on jack stands.  X Research source
Empty the radiator and loosen the seal to release the valve located under the front passenger's side.  X Research source
Disconnect and remove the A/C heater drain tube from the firewall, if the car is equipped with air conditioning.
Detach the hoses from the heater core by releasing the clamps. After the cooling system has had a chance to completely empty, lower the jack.  X Research source
Determine the access area for your vehicle. In the case of a Cavalier, you'll have to remove the entire instrument panel from the dashboard.
Examine the placement of the pipes surrounding the heater core once it is exposed.
Disconnect the heater core outlet.
Detach hoses that may still be installed at the heater core, loosen the bolt from the heater core cover and remove the cover.  X Research source
Release the heater core mounting clamps and dislodge the heater core.
Wipe up any fluid which may have leaked.
Remove any seals, O-rings or other components that you can utilize on the new heater core.
Install these parts on the replacement heater core.
Position the replacement heater core and attach the mounting clamps.
Replace the heater core cover and reconnect the heater core outlet.
Reconnect any hoses that you can reach from the interior of the car.
Wait to reconnect the instrument panel until you've confirmed that everything is properly installed with no leaks.
Jack the front end up again and replace the jack stands.
Reconnect the remaining heater core hoses.
Replace the A/C heater drain tubes at the firewall.
Follow the owner's manual concerning the proper coolant combination and seal the valve and refill the radiator.
Remove the jack stands and let down the car. Reattach the negative battery cable.  X Research source
Confirm that the hoses are properly reattached and that the radiator is both filled and capped.
Start the car and examine the area surrounding the heater core, checking for leaks.  X Research source
Allow the car to run until the engine has reached full temperature, and then continue to look for leaks.  X Research source
Start the heater and confirm proper function.
Replace the instrument panel after you've made any necessary hose reinstallation.
Confirm that the radiator is full according to the instructions in the owner's manual.
Verify that each of the removed components is correctly reinstalled.
Dispose of all of the fluids you have drained.  X Research source