I can try getting a good answer for you after supper (this evening) if no one else knows off the top of their head.
They may have cut off the speaker connectors by the looks of it. Can you tear that gray foam tape off and post pictures again?
It'll end up being 2Xtwo wire connectors, i can get the wire colors for you. They then feed into the fader in the center console.

If you look at those 2 black connectors (in the #2 image you posted, the left and right circles), you can see on the back of the original radio where these plug in, neither of these are for the speakers. The speakers are just 2 terminals each, one of the harnesses on the car is white, the other is black, and you'll see where each color goes on the back of the radio (color coded).
Those 2 black connectors are for power, ground, lighting, etc. I can tell you what each wire does if you need to know. I believe this is for your 89, right? I know you don't need a radio code, and I don't believe anything special at all will be required to make the radio work again.

Gilly