99 Blazer heater core replacement

Just replaced the heater core in my 99 4X4 Blazer. Learned a few things, so I’ll post the step by step.

First, I read (for once) the info that came with the replacement core. Too bad I didn’t read it first. I should have flushed the system before installing the new core. I actually thought of that after I had started, but by that time it was too late. Of course, if you had a mongo leak in the heater core you wouldn’t have been able to do it anyway, but I only had a small leak, so I could have. I also learned that an aftermarket installation of an improperly grounded device (radio, snowplow, etc.) can cause electrolysis that will destroy your aluminum heater core and radiator , so word to the wise when you install things like that. So here goes:

1) Take the battery out so you can disconnect the lower radiator hose and drain the system. Might as well disconnect the heater hoses now, too, so they can drain.
2) While that’s doing its thing, start on the dash. You’ll need to remove all the front and lower panels, the radio, the heater controls, the 4X4 control panel, rear wiper control, glove box, ashtray, brake release, etc. I used a couple of screwdrivers to carefully remove the defroster panel to get to the 3 bolts back there that hold the dash on. You also need to remove the speaker covers to get to one bolt under each that holds the dash on there. So there are a total of 5 bolts holding the top of the dash to the firewall. Leave the middle one attached, and remove the rest.
3) You’ll have to disconnect a pantload of connectors, including the main electrical supply behind the glove box. It’s got one bolt in the middle, then you have to gently pry it back out. There’s a connector connecting the vacuum lines coming from the dash controls to the heater box that has to be undone. You basically have to disconnect every connector you see. I found it easier to take the little things (courtesy lamps, etc.) off the dash altogether rather than trying to undo those tiny little connectors. My car has a center console that I unbolted and moved back since I thought it would be easier to get the heater box out. I don’t think you really need to, but it’s only 3 bolts (one under the rubber liner in the front, and 2 under the container under the armrest).
4) Alongside the steering column are two aluminum L brackets – 4 bolts each. Take them off. If your car has the adjustable steering wheel, lower it as far as it goes. Move the shift lever into L (as far down as it goes). There’s another bolt behind the glove box (easy to get to now that the glove box is out) that holds the dash on to the body panel. There may be one or two other bolts, but I think that’s it. You’ll see 2 pins on the right and left body panels that hold the bottom of the dash up on its hooks. Go back and (with pressure against the dash – toward the front of the car) remove the one bolt at the top holding the top of the dash to the firewall. As you let go, the dash will tilt forward.
5) You’ll now be able to fish some of the cable harnesses back through their holes and undo any connectors you may have missed.
6) With a friend, lift the dash up and over the steering wheel. If you have the adjustable wheel, it should go right over. If not, you may need to drop the column. Be careful in case you forgot any vacuum lines or wires that might be still connected.
7) With the dash out, you’ll be able to see the heater box. Bend the passenger side carpet back a little and you’ll see a bolt attaching the heater box to the firewall. Remove it. On either side of this bolt are 2 tabs with bolts coming in from the engine compartment. Every other post here says to just break or cut the tabs. I agree. The alternative is to get at the bolts from the engine compartment and it’s friggin impossible. Even the dealers break them. Now get under the hood.
8) There are 3 bolts and 1 nut holding the heater core from the engine compartment. The nut is next to the hoses and is the easiest thing about this project. Take it off. To the left of the hoses and below them, you’ll see a 10 mm bolt holding the A/C compartment cover onto the firewall. This also connects to the heater box, so it has to come out. There’s another one on the top of that cover and further left. Take that one out, too. The third is a pain. It’s inside the A/C compartment. On the top of the cover is printed “cut here for service.” The whole housing cover is a soft plastic material, and you can cut it easily enough with a razor knife, but I found it was easier to just undo the cover and move it out of the way. There are six 7/32" bolts holding the 2 pieces of the plastic cover to each other. Remove the top 2 and the 2 closest to the front of the car. You'll be able to bend the plastic cover out of the way. Disconnect the electrical connector, then remove the 3 small bolts holding the blower resistor in place and remove it. You'll be able to get a 10 mm universal socket on a long extension down into the A/C compartment and remove the last bolt. Just be careful not to drop it, although if you did, you'd be able to get it out once you remove the heater box anyway. There are great pics of this part here: http://www.automotiveforums.com/vbulletin/showthread.php?t=368427&page=2 (http://www.automotiveforums.com/vbulletin/showthread.php?t=368427&page=2).
9) Replace your core and but everything back. I got mine at NAPA. It didn’t come with foam to replace the foam that was all around the old one. I started to take it off the old one to put it on the new one, but gave up. It was falling apart. I don’t think it will matter – it’s in there pretty tight cuz it’s not exactly the same as the OEM. It’s a little taller, which works out since the cover prevents it from moving around.
10) To get the heater box back on, I just got it positioned over the lower outlet and worked the necks and long bolt back through the firewall.

Hope that helps!

You forgot one important point.
Before removing the dash or dropping the steering wheel, I would first defuse from the fusebox and then disconnect the airbag system connectors at both the steering wheel and glovebox side.

I'm not sure that this step is relavent since you have removed the battery, but if any charge remains loaded in this system I would recommend to caution on the safe side.

When I did mine I didn't have to lift the dash over the steering wheel. I just moved the glove box side out of the way ... This was enough.

Good point you made ... re-emphasizing the need to cut or break the tabs on the lower part of the heater compartment. In any do-it-yourself manual, this step is ommited. You would be absolutely lost without this advice. I did it from a manual, step-by-step, got to this step and was stuck with nowhere to go. A quick reference to this sites recommendations gave me the break the tabs answer I needed to proceed.