this is one hard job and most dealers will charge you about 400 - 500 $ plus shop supplies and god knows what else.
1. take the day off and don't drink beer while doing the job, it will take till sun down if you do.
2. you will need some tools such as 6 mm and 7 mm sockets and drive ext are nice but the best would be a flex shaft screw driver
check out this article as it is better that me writing an essay
here is the link
Ok, so the first thing to do is remove the one heater core box bolt that is under the dash on the passanger side behind the carpet and cut the two black taps with a dremmal on eithe sided of that bolt. Next remove all the undash plastic, glove box, front dash cover and insterment cluster. Next on the driversside there are two 90deg angel brackets under the stearing colum remove them (4 screws) also there are two screws on the lower part of the dash one one each side near the floor next to the door that need to be removed. After that you need to remove the speaker covers and the defrost vent cover to access the screws that will lossen the dash I think there are 6 screws. Once they are removed the dash will roll back. You need to disconnect all the wire harneses that are connecting the dash to the fire wall so the dash can be pulled out farther. (I found that tilting the dash just did not work there was not enough room to work.)
Ok time to remove the heater core in the engine compartment there are two bolts and one nut. The nut is right next to the hoses that go to the heater core. One bolt is just to the left of the distributor below the heater core supply hoses. the other bolt is the hardest bolt to get to it is in the A/C evaporatior box where it says cut here for service so get in there and lossen that bolt and the heater box comes right out on top of the hater box there are two screws that are holding a cover over the hater core remove them and that is the core. To reassemble just do it all again backwards.
The total project time for me was 5 hours. It would have gone alot faster if I had some swivel sockets and longer extensions. I you. WHAT EVER YOU DO DO NOT ATTEMPT TO SPLIT THE HEATER BOX THERE ARE SCREWS GOING ALL AROUND THE BOX AND YOU WILL MORE THAN LIKLEY BREAK THE BOX AND AHVE TO REPLACE IT. THERE ARE PEOPLE SAYING TO SPLIT THE BOX ON SOME POSTS BUT THAT DOSENOT WORK ON A 1998 BLAZER.
I DID 4 DAYS WORK OF RESEARCH TO FIGURE OUT HOW TO DO THIS REPLACMENT.
how to replace a heater core in a 1997 Chevy blazer
how do i replace a heater core in a 2001 Chevrolet blazer
You can but you still won't be able to get it out.
Drain the water from your 1994 Chevrolet Blazer cooling system. Remove the heater core water supply hoses. Remove the heater core retaining bolts. Reverse the process to install the new heater core.
You have to completely remove the dash to get access to the heater core.
replace heater core in a 1991 Chevy work truck 1500
with your hands
Detach the screw that attaches the heater duct on a Chevy Cavalier and remove the heater core. Once this is done you can replace the faulty heater core with a functional one.
pull the handle down
the heater core, heater core hoses or heater control valve is leaking
How do I replace the heater core in a Chevy express 1500