Heater core is located under the dash board, as Jenkins said you need to remove the dash board amd all other panel to access it, you will have to spend at least 2 hours to remove them if you're not familiar with it, becareful not to break plastic locks and tabs. Get your self a service manual for your car and it will be a great help.
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Suggestion: see if the blend door opens or closes completely to allow all air through the heater core. To see the blend door moving, remove the panel that says "Cut here for service" under the hood along the firewall. I have a 99 Bravada where the blend door keeps moving back and forth constantly. Changed the blend door actuator but still keeps moving. The air inside is only warm since the door keeps allowing bypassed air through.
UNHOOK BOTH HEATER HOSES AND FLUSH OUT HEATER CORE
DO IT FOWARD AND BACK FLUSH / COVER DIST. CAP OR YOU WILL
FILL IT WITH WATER / IF IT CLEARS OUT HOOK HOSES BACK UP AND
BLEED AIR OUT OF SYSTEM / IF IT DOES'T CLEAR OUT HEATER CORE
WILL NEED TO BE REPLACED (DASH COMES OUT TO DO HEATER CORE)
In order to replace a heater core in a 1998 gmc jimmy the coolant must be completely drained and the entire dash including radio, vents and panel covers must be removed. four screws hold the top portion(closest to the windsheild) in place. they can be found under the windshield defrost vent. after the entire dash is removed a safety harness under the steering column must be removed and the steering column must be dropped to the driver seat. The heater core housing(black plastic) is located right center of the dash and is a two part housing one piece on top of the other. the housing is attached to the fire wall and must detached before replacement can continue. after detaching the housing from the fire wall separate the housing by unscrewing the six screws that hold it together. the heater core is now exposed. disconnect the heater core from the two hoses and discard. installation is the un installation process in reverse. Good luck, it is a very long and drawn out process that is not accomplished in just an hour or two . make sure you have most of the day or several afternoons to finish . i started this and it took me 2 afternoons to take apart and put back together
THE STEERING COLUMN (DO NOT ROTATE THE STEERING WHEEL AFTER THE INTERMEDIATE SHAFT IS REMOVE, YOU WILL BREAK THE SIR COIL), WINDSHIELD MOLDINGS, AND DASH HAVE TO BE REMOVED FIRST. UNDER THE HOOD THE BLOWER RESISTOR, R/H INNER FENDER WELL, HEATER HOSES TO THE HEATER CORE AND SEVERAL NUTS AND BOLTS HOLDING THE HEATER PLENUM TO THE FIRE WALL INCLUDING THE HIDDEN BOLT UNDER THE BLOWER RESISTOR IN THE EVAPORATOR CORE AREA. THEN FINALLY THE BOLTS ON THE INSIDE OF THE TRUCK HOLDING THE PLENUM TO THE FIREWALL.
NO DISRESPECT INTENDED BUT IF YOUR NOT A TALENTED MECHANIC I WOULD FIND ONE OR HIRE ONE THAT CAN DO THE JOB FOR YOU. THIS IS QUITE INVOLVED FOR A NOVICE.