I replaced my heater core last night. It was not easy, but I did do it all by myself with no help, and it took 4 hours. I am going to outline the steps below for others. This was in my 1985 IROC with no air.

I drained the coolant from the radiator by removing the plug. I left the cap on so the water would trickle out and was manageable rather than just gushing out and making a mess. This went smoothly.

I removed the back TPI distributor cover. I then unhooked the heater hoses. The return hose came right off with no problem, but I did have to cut the other hose off. No big deal as there was still enough hose left to reuse.

I removed the under dash panel on the passenger side.

I then removed 4 of the 5 access screws to the heater core cover. The 5th screw was an absolute nightmare. I had to take off the dash pad. Remove the top dash mount nut and bolt under the dash and pull the dash out about 3 inches, remove the speaker, and the vent ducting to get to it. After that, it was pretty easy .

I then removed the bracket that holds the heater core. Just remove the lower one and then loosen the top one so you can turn the bracket out of the way, there is no reason to remove the top one, that is just extra work.

With one good tug out came the heater core. My new heater core tubes were pretty far off from the way the old ones were oriented. GENTLY bend your tubes until they are as close as you can get from eyeballing to the original ones. THIS WILL SAVE MAJOR FRUSTRATION! With a light you can see up into the box as you are putting the heater core in and see how far off your tubes are from going through the holes. It took me about 8 tries but then it did slip right into place.

Assembly is the opposite of teardown. A few comments. I have read where some people say they did not have to pull the dash out. No way. It only took 15 minutes to take the dash pad off, unbolt the dash and pull it out and remove the speaker and ducting. I tried to get that damn bolt for over a half hour before I finally removed the dash etc. Also, remove the ecm and set it aside. It is in the way and while this job could be done with it in place, it is a nuicance and life will be easier with it out of the way. Hopefully this helps someone out. I have no doubt I could do the job again in about 2 hours now that I know all the steps. The dealership charges about $550 and that does not include the new heater core. It is worth doing it yourself, just be patient.

i never had to take the dash pad off, i used a combination of 1/4" drive wrenches and swivels - 86SC

Also,with a trans am with a dash pouch is is way easier. If you remove the dash pouch, there is a hole that you can access the infamous 5th screw from., with a long 1/4 extension. - Toehead