Well I'm making a writeup on my successful heatercore swap. Was a pain in the ass but I've been more frustrated with other jobs....
Remove center console. If you don't have one, you're lucky. Mainly because it would have been easier working without the shifter on the floor.
Step 2: Remove the carpet stuff under the dash (oh hey look, there's my speaker crossover.. lol)
Step 3: Unscrew and remove duct to floor.
Step 4: Remove floor duct pan.
*note: Try to undo the clips that hold the big mess or wires, you'll be able to pull the parts though easier*
Step 5: Remove heatercore plastic cover.
Note: For the back screws, you will need to use a small wratchet, and deep socket.
Oh snap, coolant!
Step 6: You should have put a rag down to catch coolant, now it's on the carpet.
Pics of heatercore:
Pipes from heatercore going into firewall:
Small drip of coolant from corner:
Pic of pipes coming out of the firewall in the engine bay. (Top one is disconnected in pic)
Step 7: Remove NFG heatercore. Disconnect rubber hoses in firewall, remove the 2 screws from the inside.. it pops right out.
Pics of NEW heatercore. Note: unlike the factory one, this one has the metal tubes connected. I got this from GM dealership, unless they got it from NAPA or something.
Step 8: Install heatercore, installation is reverse of dissassemble.
Note: If you have good coolant, don't bother to drain it. When you disconnect the rubber hoses from the firewall, minimal coolant will come out. Have a couple rags handy and on the floor if you don't want to make a mess.
I had put Stop-leak in prior, which didn't work, so I flushed it well before installing the new heatercore.
Bars stop-leak. Most of this mess never made it to the heatercore, it just sat in the coolant reservoir. If you decide to ever use this crap, make sure you add it to a COLD radiator so it enters the system. Dumping this out it looked like sand in the bottem... YUCK.
This was a fairly easy job on a 3100 engine. I don't think I would recommend you do it yourself on a 3.4DOHC if you are a beginner, cause getting to those rubber hoses on the firewall would be a major PITA.