These radios came with a five digit code as an anti-theft feature. You have to enter the code after you disconnect the battery. Originally the car would have come with a small card that had the number on it. If you got any original paperwork or manuals with the car, it might be in there.
You can get the code from your local dealership. They'll find the serial number of the radio and the VIN of the car, and look up the code.
Another thing to try is to look at the fuse box cover or inside the glove box. You're looking for something that says "RC xxxxx" where the RC stands for radio code and the xxxxx is the code. Some dealers marked the cars with the codes before they went out, so they didn't have to look them up each time the car came in for service.