how do you replace heater core

You will want to get a Haynes manual. This is a very involved job, most of the dash has to come out.

Buy a Haynes manual, you can get it at any auto parts stores. In it will have detailed instructions. The job is a bit of a pain in the butt. there are a couple of hoses on the firewall and the rest of the work is under the dash (by the glove box over to the middle) a bunch of bolts and connectors. There will be some spillage of coolant and elbow greese. It is a back "hurting" jobs. If you have never done one before, you might want to take it in. As I said earlier, in the Haynes manual, it will have detailed instructions. If you go DIY, remember to drain the coolant, remove rad cap, follow the heater coolant hoses coming off the engine and remove them (tag were they go, you will NOT want to mix up the order)(black rubber hoses running to the firewall). Then got inside the car, under the glove box, remove all the access panels. in the middle/passenger side you will see a extra encasing, remmove the cover, you will see a "brass/copper" rad-like square, unhook the connectors and hold-downs, There is very little room to work in here and you will be laying on your back as well as working by feel, wiggle the old unit free and remove. The install is the apposite of the removal. Do your best to not damage the new unit and dont twist it!! when you are putting the new unit back in, there will be a point where you will think that if you just give it a twist it will go right in, yes it will but, everything else may not (90% chance) bolt back up and you will have a leak. Best of luck and read the directions over a few times and go slow.

I've only replaced one heater core in my life. I found that the Mitchell manuals are very complete for this job. You can go to a public library an photocopy all the info. Make sure you have all the tools needed by reading through the procedure first. You may well have to discharge the ac system so that would be the FIRST thing done.

The heater core is located in the center of the firewall. You begin by removing the first cover to the ductwork located behind the heater controls. This has two screws in it that are barely visable. There is a round hose on each side of the front that you will have to detach by pulling first then move it forward and pull to the passenger side. Once this is removed look for the 4 screws that hold the cover on the heater core remove those and then pull the front side down and pull toward the seats. Now jack the car up and use jack stands. You will have to get the car high enough to get under it and reach up in behind the engine. Once the car is secure remove the brace behind the transmission that goes from one side of the frame to the other it is fastend with 2 bolts on each side. This will allow you enough room to reach the inlet and outlet hoses to the heater core. Remove the clamps using a channel lock pliers and remove the hoses. Once hoses are off go back inside and remove two screws from heater core brackets on passenger side of heater core. Pull the heater core out the passenger side. You do not need to remove the dash. This procedure will take 1.5 to 2 hours to complete. :)

Take it to a GM dealership and let them handle it. I had this problem and trust me, it's something you don't want to do if you have to ask how to do it. Best left to a professional.