Interesting info... A 79 Camaro never came with two front speakers
So, two of those slots in the middle plug (5-8) would be empty.
They only had two rear speakers with the U80 option.
Same deal on that plug (9-12)... Two empty slots on the speaker harness if it only had a single rear speaker.
If the radio powers up... You apparently have the plug with connections 1-4.
That plug should include a grey wire for illumination and brightness would be controlled by the rheostat in the headlight switch.