Helped a friend replace the heater core on a '99 Blazer this weekend - and it sucked hard. We figured out why shops charge $1k for this job (at least here in urban hell).
I write a short synopsis here, so that if someone encounters the same issue it'll turn up in a search. It's apparently a fairly common problem with these vehicles.
All you really need are some 1/4" ratchets with 7mm, 10mm, 7/32" sockets, a trim removal tool, screwdrivers, razor knife, and pliers.
Step 1: Disconnect the battery. Remove ALL the kick panels, trim panels, instrument panels, glovebox, stereo, climate controls, light switches from the front end of the vehicle.
Step 2: Disconnect every connector behind the dash (both vacuum and electrical).
Step 3: Pop the defroster vent grill and speaker covers off. Undo all 5 of the screws (7mm heads) that attach the top end of the dash to the chassis. Undo all the 10mm bolts below/behind the dash that hold it to the chassis (there's 3 or 4). Put the gearshift in L and tilt the wheel all the way down. Remove the entire dash assembly. You'll need a partner for this.
Step 4: Remove the heater hoses at the firewall. You won't lose hardly any fluid if the vehicle's level. I suggest corking/capping the core tubes at this point.
Step 5: There are five bolts and one nut holding the heater box assembly to the firewall. These are all 10mm and all accessed from the engine bay. There is no practical way to get at the lower two, so the best option is to cut the plastic tabs off the bottom of the heater box from the cab using a Dremel. Every writeup I've found advises this, and I learned that even the dealer do this instead of trying to undo the bolts. There are two bolts and one nut at the top of the heater box that are easily undone from under the hood. The two bolts just look like they hold the AC condenser cover to the firewall. The nut is on a stud that pokes thru the firewall right by the trans dipstick. There is one last bolt, and it's a PITA. Cut the rubber condenser covering surrounding the blower resistor assembly with a razor knife (the thing with the electrical connector on top of the condenser housing is the resistor assy.). Remove the rubber and discard. Undo the three 7/32" screws holding the resistor assembly and remove it. Look down the hole that you just oncovered and you'll see one last 10mm bolt head in the firewall. After trying different routes, we found one that made it relatively easy: cut a hole in the condenser cover using a dremel straight out from where that bolt is. Then, use a long extension (about 12") and go straight at the bolt. Have your buddy pull back on the heater box so the bolt stays in the hole.
Step 6: The core can be accessed by rocking the heater box back and removing the plastic core cover (three 7/32" screws). Simply slide the core up and you've got it.
Step 7: Reverse all the above things you just did to put it back together. I suggest getting a piece of scrap plastic and bonding it over the hole you cut in the condenser cover.
I write a short synopsis here, so that if someone encounters the same issue it'll turn up in a search. It's apparently a fairly common problem with these vehicles.
All you really need are some 1/4" ratchets with 7mm, 10mm, 7/32" sockets, a trim removal tool, screwdrivers, razor knife, and pliers.
Step 1: Disconnect the battery. Remove ALL the kick panels, trim panels, instrument panels, glovebox, stereo, climate controls, light switches from the front end of the vehicle.
Step 2: Disconnect every connector behind the dash (both vacuum and electrical).
Step 3: Pop the defroster vent grill and speaker covers off. Undo all 5 of the screws (7mm heads) that attach the top end of the dash to the chassis. Undo all the 10mm bolts below/behind the dash that hold it to the chassis (there's 3 or 4). Put the gearshift in L and tilt the wheel all the way down. Remove the entire dash assembly. You'll need a partner for this.
Step 4: Remove the heater hoses at the firewall. You won't lose hardly any fluid if the vehicle's level. I suggest corking/capping the core tubes at this point.
Step 5: There are five bolts and one nut holding the heater box assembly to the firewall. These are all 10mm and all accessed from the engine bay. There is no practical way to get at the lower two, so the best option is to cut the plastic tabs off the bottom of the heater box from the cab using a Dremel. Every writeup I've found advises this, and I learned that even the dealer do this instead of trying to undo the bolts. There are two bolts and one nut at the top of the heater box that are easily undone from under the hood. The two bolts just look like they hold the AC condenser cover to the firewall. The nut is on a stud that pokes thru the firewall right by the trans dipstick. There is one last bolt, and it's a PITA. Cut the rubber condenser covering surrounding the blower resistor assembly with a razor knife (the thing with the electrical connector on top of the condenser housing is the resistor assy.). Remove the rubber and discard. Undo the three 7/32" screws holding the resistor assembly and remove it. Look down the hole that you just oncovered and you'll see one last 10mm bolt head in the firewall. After trying different routes, we found one that made it relatively easy: cut a hole in the condenser cover using a dremel straight out from where that bolt is. Then, use a long extension (about 12") and go straight at the bolt. Have your buddy pull back on the heater box so the bolt stays in the hole.
Step 6: The core can be accessed by rocking the heater box back and removing the plastic core cover (three 7/32" screws). Simply slide the core up and you've got it.
Step 7: Reverse all the above things you just did to put it back together. I suggest getting a piece of scrap plastic and bonding it over the hole you cut in the condenser cover.
The S10 is a real PITA but the C5 Corvette is much worse, you have to remove the dash, center console and the upper intake.
