Whats the proceedure for replacing the heater core in a 1990 Bonneville? Any help
would be greatly appreciated.

Thanks
Bonnie-vill

Your in luck , it is pretty simple as you can get to it from behind glove box.Take off glove box and hush panel and you will see black plastic heater box, it is in there.You will probably have to remove HVAC programmer and actuator along with other screws if I remember correctly.The hardest part is the 2 heater hoses in the firewall , they are a bitch to get at.
What is the reason u are replacing?

I changed the heater core in my 1990 Bonneville this winter. It was pretty easy and went well except I got the wrong heater core from the parts store. On cars built later in the model year like mine, A different heater core was used. The earlier one had one of the core outlet nipples angled. The one I took out of my car had both nipples going straight out of the core. As mentioned previously, access to the hose clamps on the engine side of the firewall is very tight. It may be difficult to determine which core you have, so try to line up a ride back to the parts store before you get it apart. Fortunately, I had my old faithful 1976 Lemans Sport Coupe to drive when I got the wrong core.
When you install the new core, use a traditional screw type hose clamp on the hoses instead of the original equipment spring type clamp. You'll understand why as you attempt to get the spring clamps off. To remove them I used a six inch needle nosed vise grip. There is a specific tool made to remove and install spring type hose clamps in tight spaces, but since I planned to use the screw type clamps instead, I didn't feel the need to spend the money.
There are several small hex headed trim screws that will need to be removed once you get the glove box off. Keep them separated as to which screw goes into which piece of trim. They look very similar, but have different thread pitches. I found three different threaded screws out of the twenty or so I had to remove while replacing my heater core. If you have auto climate control, be careful about unhooking the control rod from the flapper box, I broke mine but was able to repair it. Just a heads up.
Check the condition of the heater hoses while you have them unhooked, this would be a great time to replace them.
The rest of the swap is pretty straightforward, take the old one out, put the new one in. If you have any more specific questions, just ask.

The original ones have plastic sides on em. I just took a screwdriver, and broke the upper connector. I then tookthe core, bent it downward, and it broke right out of there.

Thing is, i'd spent an hour trying to take the hoses off, when i decided to do this, out of frustration.. ARGH!
Screw type hose clamps recommended. Jack the front end up to bleed cooling system of the air in the core.

It took me more time to find my small needle nosed vise grip pliers than it did to remove the clamps with them. Maybe ten minutes tops.
Been there done that on the frustration thing, though. I wasn't in any hurry and spent a minute or two looking at it and figuring it all out. I've found that'* usually better than breaking things. To each his own.
As far as a head start on the procedure,
1. drain radiator through small valve on lower left side.
2. take off the firewall cover on the right side of engine compartment
3. remove trim panel piece under right side of dash.
4. remove glove box.
5. remove climate control programmer. might need to use a small screwdriver to pry off wiiring connector, don't undo vacuum lines, they will have enough slack that you can move them aside far enough to
6. remove the black core housing.
7. attempt to remove spring type hose clamps.
8. cuss a lot because they are so hard to reach.
9. figure out I was right and go find a small needle nosed vise grip to remove the clamps.
10. remove the heater core.
11. install new heater core.
12. replace everything you removed, and fix everything you broke, unless you replaced it.

please put your year and trim level (le, se, sse) in your signature, it helps us members help you, considering theres a difference in the repair procedure if you have the automatic climate controls (sse model only) or the manual controls (le, se)

I am replacing the core because it is leaking. The car does have SSE.
Thanks to everyone for the help!
Bonnie-vill

My car is a 1990 LE with automatic climate control. The repair procedure only differs in that the programmer needs to have the electrical plug-in disconnected and then it must be removed from the core housing.