Procedure to replace a heater core in a 94 SeVille:
*Do not let your wife help unless you have a strong marraige!! I promise you will be at each other's throats before the project is finished.
1) Remove the negative terminal on the battery. You're going to be working around the passenger airbag and pulling electrical connectors of the A/C Programmer. 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. Next, un-clip the airbag connector that's clipped in place on the back side of the trap door. I used a spring clip tool to unhook the stainless steel clip that holds the airbag connector to the glove box trapdoor, then after the glove box is out you have more room to un-clip the airbag connector. It has double-redundancy on the connector so it does not come unclipped accidentally, and it is a little hard to take loose.
3) Remove the glove box. Six torx 15 screws hold the glove box in place. Four long ones on the front [open the door first] and two short ones on the glove box floor. I used a full-sized Torx screwdriver to remove the front four, and a small ¼” drive Torx 15 socket in a ratchet to remove the bottom ones. The three connectors on the left side are configured so that you can’t hook them up wrong. I took all of them loose and threw the glovebox in the rear seat.
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 (can use 9/32”) along the near edge. Remove these and the panel will drop down and pull out, disconnecting the courtesy lamp wire. 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) Next, 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. I took time to remove the hood. This gave me lots of headroom to get to the hoses, plus extra light. I had my son to help me. Removing the hood is definitely a two-man job. (Scribe around the hinge mounts so you retain hood alignment). My hoses were old so I replaced with new ones. There are vacuum lines and wiring that pass through the firewall right in the area where you are working on the hoses, behind the strut tower. I carefully unhooked several of the connectors and moved them back to get more clearance to access the hoses. Careful if using a crowbar to pry off the hoses, you might damage the plastic housing of the heater box that is visible under the hood, on the front side of the firewall. The heater box is two-layered, with insulation between the layers. I shattered mine and had to take time to epoxy the pieces back together. A better technique would be to take a box cutter and slice the hoses and pry the remnants off. Also, the front ends of the heater hoses are virtually inaccessible so slicing with a box cutter there is much easier than loosening the clamps, etc. Good luck.
6) Back inside the cabin take a look inside the glove box opening. Tucked away back to the right of things, against the firewall, you will see a silver metal box. This is the A/C Programmer. It is mounted to the firewall with two 10mm nuts. Carefully remove these nuts and swing the control unit down and remove the three connectors along the bottom edge of the programmer (mine were colored red, white, andd blue). They release with a clip connection on the back side - pinch and pull.
It is possible that the heater core could be replaced without removing this controller, but I was concerned with spilling water in it so I decided the risk of disconnecting the A/C control unit connectors was less risky than spilling coolant onto the circuitry.
The object here is to make room to the right side of that black plastic box you're seeing. This is the heater box that houses both the heater core and the AC evaporator core..
7) Look on top of the heater box. You will see a shiny silver control rod. Un-clip this on the left side, where it is held in place with a yellow plastic clip. All you need is a flat screwdriver under the rod on the left end and twist to pop it out of its retainer. This control rod swings a baffle through an arc of about 45 degrees to change the airflow when heat vs cool is selected. This rod is threaded on the right-hand end and should be carefully adjusted to completely close the baffle as the control rod moves back and forth. (On reassembly I used my left hand to swing the baffle and my right to twist the rod and set the adjustment correctly, then pressed the rod down into the retainer clip). You have to unclip this rod and then remove the control box mounted on the right-hand end of the heater box. This assembly is held in place by one 9/32” hex-headed screw on the front of the assembly. The rear edge of this assembly has two tabs that protrude towards the firewall and engage two little slots in the back (nearest the firewall) of the heater box. Remove the screw and swing the assembly to the right, then drop it down and out. It has the colored air/vacuum hoses attached to the control circuitry. Do not remove these. Just carefully lay the box against the center console and try not to spill your beer in it.
8) The remainder of the heater box is split vertically in its center and is held together by two (9/32” I think) screw nuts. One you can see by looking through the glove box opening, the other is about four inches below it. Remove these and open the heater box to the right to expose the heater core. I had to swing the front of this cover all the way to the right, then slide it down to get it out.
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 foam air seals. Looking inside the box, where the core was you will see four strips of foam surrounding the opening. These should be replaced. I used a small flat screwdriver to scrape the adhesive backing loose, and then strip it off the heater box. It's tight in there, but take your time.
11) Install the new core once the foam is 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) Reassembly is the reverse of taking it apart. If you got this far, you should be able to get Humpty back together again.