http://www.coloradok5.com/heatercore.shtml Check this out its a very helpful writeup
You have to completely remove the dash to get access to the heater core.
You have to remove the entire dash board to gain access to the heater core.
with your hands
how do i replace a heater core in a 2001 Chevrolet blazer
how to replace a heater core in a 1997 Chevy blazer
Unfortunately, there is no "easy" way to change the heater core. The entire dash board has to come out. then it's easy. Then you have to put the dash back in. Sorry :(
Disassemble the dash and open up the HVAC compartment.
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.
The heater core is inside the heater unit on the passenger side under the dashboard
The heater core has no gasket. If it is leaking, it must be replaced.
with your hands
To many steps to list here, would recomend purchasing a shop manual if you intend on replacing that heater core yourself.