i have a '91 sonoma...it was a mess when i got it. i snipped the wires that were there and wired em to the wiring harness that cames with the radio, and BAM! i have a cd player. if u want, pm me and ill give u a step by step
If the plug is gone, your gonna have to test for wires. All GM cars are the same, and are as followed.
Orange- 12v constant
Yellow - 12v switched
Black - Ground
Front left speaker + tan
Front left speaker - gray
Front right speaker + light green
Front right speaker - dark green
Rear left speaker + brown
Rear left speaker - yellow
Rear right speaker + dark blue
Rear right speaker - light blue
Alot of times when the plug is cut off and you have to hard wire the radio in, some of the fuses are cooked and you wont read any power. Make sure you test ALL OF THE FUSES if for some reason you cant find constant power or switched power.
The power wire that went to the factory radio is enough power to run just about any radio out there today, unless you have one of the 60x4 Alpine decks. If thats the case, run the wire straight to the battery, or grab power off the large gauge red wire at the steering column.