Replacing a heater core on any vehicle is a difficult process, since you have to completely remove the dashboard to get to it in most cases. It is a bit more difficult on a 2000 GMC Sonoma, because you have to completely remove the truck's heating/ventilation module after removing the dash in order to change the core. If your heater is not producing any hot air and the coolant is actually leaking into the truck, you likely need to replace the core.
Disconnect the truck's negative battery cable.
Open the drain plug on the radiator and let the engine coolant pour into a large container. Repeat the process with the same container at the drain plug for the engine block.
Grasp and detach the inlet and outlet heater tubes from the engine firewall. Plug the fittings to avoid contamination.
Unclip and unscrew the lower dash panels with a screwdriver, then unscrew the radio from the dash with a wrench and pull it out, disconnecting the antenna lead and electrical connector.
Unscrew and remove the bolts for the pivot bracket on each end using your wrench, then remove the four nuts for the steering column and lower the column. Remove the single bolt on the steel pencil brace located near the center bottom of the dash.
Pry out the speaker and defroster grilles from the dash with a screwdriver or trim stick, and remove the bolts behind them.
Grab the dashboard and pivot it to the rear; disconnect all electrical connectors behind it, then pull the dash out of the truck.
Disconnect all the vacuum and electrical connectors from the heating and ventilation module, making sure you know where each connector goes.
Unscrew and remove the blower motor resistor from the firewall on the engine side; it has three mounting screws requiring a screwdriver. Remove the one heating module bolt inside the resistor hole using a ratchet and extension with a flashlight's help.
Remove the other bolts for the heating/ventilation module from inside the truck and remove the module.
Unscrew and remove the top heater core cover and then slide the core out of the module.
Slide the replacement core into the module and reconnect the cover with its screws.
Install the heating/ventilation module back in the truck with its mounting bolts. Install the bolt that was within the resistor hole in that same location. Reconnect the electrical/vacuum connectors.
Reinstall the dashboard assembly, placing it on its pivots, connecting all the electrical connectors, pivoting the dash forward and connecting all the bolts, nuts and fasteners--including the ones underneath the grilles and the one at the pencil brace.
Position the steering column back in place and secure it with its nuts.
Install the radio, connecting the antenna and electrical wires and applying its mounting fasteners with your wrench.
Secure the instrument panel with the bolts and Torx screws at the pivot bracket, then clip the lower trim panels in place and apply their screws with the screwdriver.
Refill the engine coolant, using a fresh 50/50 combination of water and antifreeze if the old mixture is contaminated.
Reconnect the negative battery cable.
