Procedure to replace a heater core in a 96DeVille:
1) Remove the negative terminal on the battery. You're going to be working around the passanger airbag and pulling electrical connectors of the A/C Programer. Good idea to kill the power.
2) Open glove box and observe the access door in the rear. You are going to open this. Give a good tug on the left end of the door and pull straight in. Give it a good one, it will pop open. Un-clip the airbag connector that's clipped in place.
3) Remove the glove box. Six torque screws hold the glove box in place. Four on the front [open the door first] and two on the glove box floor. If you don't have appropriate torque drive a hex drive will work [especially on those two on the box floor]. Don't worry about the electricl wires plgged into the left end of the box. Pull straight out and then rotate the assembly off to the left out of the way - no need to unhook all that stuff and tempt Murphy.
Nice thing is that you will be working though this opening and not standing on your head under the dash, most of the time!
4) Under the dash panel is a black plastic sound panel that needs to be removed. Its held in place by three 7mm screw nuts along the near edge. Remove these and the panel will drop down and pull out. If your old core sprung a leak this caught some of the coolant - take care and wash the foam on the rear of the panel. Hot water and soap in the kitchen sink will do.
5) You are going to want to protect the left front fender while you attempt pulling and installing the heater hoses. Once done, pull the heater hoses on the engine side of the fire wall. This is a pain in that there's hardly enough room and it is a two handed operation. Take care not to damage the hoses - new sets are dear. Also watch out for all those vacume lines and electric lines that are right there crowded in and around. Keep in mind you are going to hook these up to the new core. That will prove more difficult.
6) Back inside the cabin take a look inside the glove box opening. Tucked away back to the right of things, way up there, you will see a silver box with colored vacume lines attached to its bracket. This is the A/C Programer. Look closely and you will see a single screw nut on the right end of the bracket. This is what you need to remove. Take care and do not drop that screw nut. You will never find it and will need to replace or, as I did, improvise to get it back together.
With the screw nut removed the whole A/C Programer unit wil be free and you will be able to wiggle it out and down. Move connectors as needed and take care with those vacume lines. Do not try to pull these. Don't want to think about trying to replace that stuff and remember Murphy. With the unit down flip it and pull the electric connectors off the progamers bottom edge. They release with a clip connection on the back side - pinch and pull.
Object here is to make room to the right side of that black plastic box you're seeing. This is the heater box.
7) Look on top of the heater box. You will see a control rod. Un-clip this on the right side, where it clips onto a white plastic control piece. All you need is a screwdriver under the rod on the right end and twist to pop it out of its groved retainer.
8) The heater box is split vertically in its center and is held together by two screw nuts. One you can see by looking through the glove box opening, the other is about six inches below it. Remove these and open the heater box to the right to expose the heater core.
9) The core is held in place by an end bracket secured by two screw nuts. Remove this bracket and give the core a good wiggle and pull it out and down into the cabin as best you can. You are pulling the hose stubs through the fire wall and it will come out.
10) The replacement core should come with a new bracket and air seals. Looking inside the box, where the core was you will see four strips of foam surrounding the opening. These should be replaced. It's tight in there, but take your time.
11) Install the new core once the air seals are replaced and secure it with the new bracket. Then go out and hook up those heater hoses. You are going to be pushing and twisting, but get them home and tighten the clamps well. With this done go back inside and give the core a good push towards the firewall. Recheck the bracket. The core should be in there firm as a rock.
12) All that's left is to put things back together in reverse of taking it apart.
Not bad 'eh? Book says two hour job, good luck. I've attached a pic showing how my core failed. Not what I expected.