'89 Park Avenue fuel door will not open

They all come in at the cluster on the driver side dash panel. Did you check the fuses and relays?

most heater cores are located under the dash, and not accessable without removing said dash. let a trained mechanic do this

Drivers side under the center of doors up behind the frame underneath the car

If you have fuel door release cable & door is open, verify that cable moves in & out when you activate it. If yes you can adjust tab that it rests on so it will release properly. Just bend tab slightly inward at first, till it releases correctly.

if I remember correctly, there is an over-ride switch in the trunk that will over-ride the dash release button. it should be near the latch.

The first thing to do is spray penetrating oil on the hinges and behind the fuel door. Then, check the cable connection at the release handle. Ifc it has become disconnected, reconnect. The handle may be damaged and may have to be replaced. Also, you may be able to open the fuel door by pulling the cable using a pair of pliers, or by slipping a thin blade of some kind behind it to push the release latch in. If you can get the door open, put as much oil as possible on the release mechanism plunger, and then work it into the mechanism by depressing the plunger repeatedly. If you are able to get the door open, the connection at the release handle is good, and the plunger seems to move freely, the release cable may have stretched, seized or broken, and may need to be replaced.

Check your ignition switch - not the cylinder but the switch located under the dash on the steering column. You should hear an eletric click when you turn the key.

i'm guessing you have a problem with the release cable?try slipping your credit card in between door and plunger that pops it open,the plunger is located at the rearmost centerof the door ,the door striker that rsts against plunger could just be out of adjustment,if you operate the release and see the plunger moving it's ok,adjust the striker a simple little tug towards the plunger is sometimes all it takes

are out side door handles when you push on them do they work or do they feel like nothing there very common does it have rust around handles or are they seized?