Ok, Here is a rough guide on how to change the heater core.
This is what I did and used to do my 2004 Lesabre
OEM AC Delco Part # 1560143 Heater core
Heater hoses (if you feel the need to replace them)
Astro 9409a hose clamp pliers, get the black and blue ones, they are updated strength
Door Panel remover: OEM 25313
2 Worm gear hose clamps for use on heater hoses at alternator bracket in case you break a couple of the oem spring type clamps
Needle nose pliers
1/4" drive set with metric and american sockets(sorry I cannot remember exactly which sockets I used)
3/8"x5/16" weather strip from home depot for replacing it on the air ducts along the way, I do not mention this in my the below steps.
You need at least a day to do this too!! This is a pain.
Reference this guide from the factory service manual in conjunction with my guide - just to be thorough. 2000-2005 Heater Core Instructions
1) Remove heater hoses at alternator bracket with you Astro 9409a tool. Put a bucket under your car and direct one of the hoses down to the bucket. Blow into the other hose. This will blow the coolant out of your heater core into the bucket. Now you will have minimal mess when removing the heater core.
1a)*Skip this step if you are ready to change out the heater core*
If you want to now drive your car: You may also now run a bypass hose between the heater hose fittings at the alternator bracket. You can drive your car as much as you want with no heat but can be dangerous if its cold as you have no way of dealing with ice or fog build up on your windshield. One heater hose is smaller than the other thus the fittings as well. I used my old smaller hose because it still fit on the larger fitting. I cut it at 2 bends in the hose, leaving me with 2 90degree bends. I put a brass coupler between them from homedepot, essentially making a "U" hose. Plopped that on the alternator bracket and I was good to go. This takes 4 worm gear clamps to complete and to stay together, tighten them down well. (I did this to flush out my cooling system without the heater core in the system, not completely necessary)
2) Remove heater hoses at firewall with your fancy hose clamp pliers.
3) Roll front seats back as far as they go for room for working in front of car.
4) Remove some dash panels ROUGHLY in this order(I'm going from memory):
A)Fake wood panels on either side of steering wheel with your OEM 25313 door panel remover tool. You might have to remove a the black panels at either end inside the door jam first. Start at the end of the panel at the door jam and work your way in. It just pops right out but be careful not to break it. Watch out for electrical connectors in the panels.
B)Gray panel between ash tray and glove box & gray panel underneath steering wheel.
C) The ash tray and glove box probably won't need removal but you will know later if they are in your way. Easy enough to do later if needed. Don't bother with it now.
5) Remove black panel from each footwell. It sits over your feet when you sit in the car.- Easy. Take care for any electrical wires.
6) Remove what the factory manual calls the Air distributor duct. You may have to remove a short duct piece that connects the air distributor duct down to the duct that routes air to the rear vent, and to do so you have to fold back the carpet. To fold back your carpet you must use your OEM tool to remove the plastic runners from either side of the car, it holds the carpet down. The air distributor duct is held on by a couple clips and two screws, one of the screws is near impossible to access. Don't bother trying to reinstall this screw during reassembly. At first I could not get it with my cheap-o Autocraft brand 1/4" drive set from Advance Auto. Once I tried my craftsman 1/4" drive extension and socket, I got it. The craftsman extension and socket were dimensionally smaller and were able to wedge up into the tight area and onto that bolt.
7) Remove heater core heat shield - it'* held on by screws. If you try to pull it out and it won't come out, you probably missed a screwed. I pulled it really hard and broke it because I missed a screw that I could not see.
Remove heater core cover - it'* held on by screws.
9) Now you see the heater core! It held on by one screw at the bottom. Now remove the heater core, it takes a bit of wiggling to get it out. Putting the new heater core back is not difficult either.
10) reinstall all those plastic panels.
11) install Heater hoses with your Astro 9409a tool. You need to use spring clamps at the fire wall because you cannot access worm gear clamps with a screw driver in that location. Reusing the old clamps is fine if they look fine. Worm gear clamps are usable at the alternator bracket, spring clamps are better, but worm gear clamps work fine.
12) Fill and bleed air from coolant system properly or risk destruction of your engine due to trapped air in the cooling system.