2000 Blazer I'm in the midst of replacing my heater core. Dash is removed, but darned if I can see where the air box bolts to the firewall. This is the final step to getting at the heater core so I'm pulling my hair out here.

Tyrone, Check the attached links,instruction and guides, the last link has some very good info. and pics. on S10 heater core replacement. Good luck.
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Heater core replacement 2000 S10 4 3L slideshow picture
How to Change Heater Core on 1994 Chevy 10 Blazer GMC Jimmy Oldsmobile...
98 GMC Jimmy Heater Core Replacement Video Diary
Heater core replacement procedure 99 S10

Hi
Disconnect the negative cable from the battery, then there are three studs and one bolt holding the heater box which the heater core is in there are also a few vacume lines and the heater controls that are attached to the box lwhich must be removed. There are the heater water lines that need to be removed under the hood, the studs (3) have a nut that is accesable through the hood, the bolt is accesable through the inside on the passenger side firewall. you will need to remove the airducting that is under the radio top of the engine cover and remove the bolts that hold the dash in place.However it is better to unscrew the defroster ducting on the dash.
After you remove the box then you can access the core by removing aproximatly 6 or 8 screws that hold the rear cover of the box together then you will see the core which is held down by 2 hold down brackets.
Please check this links to get a detailed idea about how to replace heater core.
http://www.ehow.com/how_5306495_change-heater-core-chevy-blazer.html

http://www.superchevy.com/tech/sucp_0707_remove_and_replace_leaking_heater_core/viewall.html
However you did not mention the model no and the year of your vehicle.But this is the general way to fix the problem in heater core.

Hope it will help.Also you can get back to me if you need further assistance.
Thankyou.

This is probably the same as it is on the S-10 truck. You'll have to remove the dash to get to the heater core box. Drain the coolant and locate the two hoses on the engine side of the fire wall and remove them. There are large electrical connectors under the dash that have to be unplugged. Remove the large bolts/nuts holding the dash to the frame and drop the steering column to ease removal of the dash. You'll be able to see the heater core box now. Remove the bolts hold it to the firewall and there are also bolts on the engine side of the firewall holding the box in. This is where it gets frustrating. Remove the blower motor resistor that is mounted in the engine bay and inside that hole is another bolt holding the core box in place. With all of the bolts out you can pull the case apart and replace the heater core. Put it all back together and fill the cooling system. Start the vehicle and run the heat on full. Leave the radiator cap off so air can escape. Watch the coolant level and add as needed until the engine reachs normal operating temperature and put the radiator cap back on. The last one of these I did I charged $450 to do plug parts. His next best quote was $950.

• Remove the front dash panel by turning the lower screws in a counter-clockwise direction, then unscrewing the single bolt on the lip of the instrument cluster and pulling the panel out. Remove the rear wiper switch and the 4x4 switches, if applicable. Lowering the tilt wheel will allow the panel to be totally removed.
• Remove the radio, headlight switch module and glove box by turning their screws in a counter-clockwise direction.
• Remove the main dashboard by unbolting it at the front, between the windshield and firewall. There are six to eight bolts holding it in, and, once they are removed, the dash can be pulled away from the firewall. Be careful not to pull too much, as the wiring harness could become damaged.
• Drain the coolant system by turning the drain plug on the lower left corner of the radiator and allowing the fluid to fall into the drain pan. Uncapping the radiator can reduce the internal pressure.
• Remove the heater core hose, on the upper left of the engine bay by pinching their hose clamps with a pair of pliers and pulling them off. Some models will have screw-type hose clamps; turning them counter-clockwise will loosen them. Age and heat may have stuck the hoses to the core nipples, and the hoses can be cut right at these nipples if necessary. Leave enough hose to reconnect the new core.
• Unbolt the heater core's mounts with a socket wrench in a counter-clockwise direction. Once the bolts are free, the heater core can be maneuvered toward the windshield and slid out. Some coolant may splash out, so have a towel handy.
• Replace the heater core with a new one by sliding it into place and securing the mounting bolts.
• Reconnect the heater core's hoses, turn the radiator's drain plug clockwise to close it, then refill the coolant system with a 50/50 percent mix of coolant and water.
• Reconnect the dashboard to the firewall, then replace the front dash panel and fasten its instrument cluster screw and bottom bolts.
• Crank up the Jimmy and add coolant until the heater core is filled and blowing hot air.
• Replace the radiator cap.

Good luck and make sure you have lots of time and use the camera on your phone and take pictures to use as reference.

Sorry, your going to have to remove the dash.

• Remove the front dash panel by turning the lower screws in a counter-clockwise direction, then unscrewing the single bolt on the lip of the instrument cluster and pulling the panel out. Remove the rear wiper switch and the 4x4 switches, if applicable. Lowering the tilt wheel will allow the panel to be totally removed.
• Remove the radio, headlight switch module and glove box by turning their screws in a counter-clockwise direction.
• Remove the main dashboard by unbolting it at the front, between the windshield and firewall. There are six to eight bolts holding it in, and, once they are removed, the dash can be pulled away from the firewall. Be careful not to pull too much, as the wiring harness could become damaged.
• Drain the coolant system by turning the drain plug on the lower left corner of the radiator and allowing the fluid to fall into the drain pan. Uncapping the radiator can reduce the internal pressure.
• Remove the heater core hose, on the upper left of the engine bay by pinching their hose clamps with a pair of pliers and pulling them off. Some models will have screw-type hose clamps; turning them counter-clockwise will loosen them. Age and heat may have stuck the hoses to the core nipples, and the hoses can be cut right at these nipples if necessary. Leave enough hose to reconnect the new core.
• Unbolt the heater core's mounts with a socket wrench in a counter-clockwise direction. Once the bolts are free, the heater core can be maneuvered toward the windshield and slid out. Some coolant may splash out, so have a towel handy.
• Replace the heater core with a new one by sliding it into place and securing the mounting bolts.
• Reconnect the heater core's hoses, turn the radiator's drain plug clockwise to close it, then refill the coolant system with a 50/50 percent mix of coolant and water.
• Reconnect the dashboard to the firewall, then replace the front dash panel and fasten its instrument cluster screw and bottom bolts.
• Crank up the Jimmy and add coolant until the heater core is filled and blowing hot air.
• Replace the radiator cap.

Good luck and make sure you have lots of time and use the camera on your cell phone and take pictures to use as reference.

stinking chevy, they put un noticable bs inside the cab. keep looking

your heater core is inside the big black box under the dash on the passenger side. you'll have to pull the dash out. then remove the black box, usually there are bolts on the firewall in the engine bay that hold it in place.disconnect the ac lines and heater hoses. remove the box and then the screws all the way around it to open it up. replace the heater core and reassemble. you'll have to recharge the freon when you are done to have ac again.

I have a 1996 blazer, tried to replace the core myself also, but the dash has to come off. Take it to the shop.

i think the inner fender and right front wheel has to come off there are bolts from under the hood that have to come outgood luck.

you have to start by taking the black covers off under the dash. And you should look under the hood to see where the heater hoses go into the fire wall. that is where the heater core is located under the dash!
When you start taking the black covers off under the dash, keep removing parts tword the core location. It will be located in an air duct sence air has to blow through it in order to blow out onto the vehicle.