Did the car originally have a monsoon or bose system? If it is monsoon like mine, I could read you off the wiring as it is in my car. I'm putting in an after market radio soon and I had to get a special module that comes with a speaker for the door chimes, and plugs directly into that plug you are missing. You could rewire an aftermarket radio then once you know which wire is which. If it'* not monsoon or bose, I would set aside your powered wires, (remote, power, ground) Should be red, yellow, and black, but not always. Then take your speaker wires and pop them with a battery by connecting the negative wire to the negative side, and the posative to the posative. I would use a 1.5V battery. That would let you know which wire set goes to which speaker.
COTY - 2012
Tom - 03' Bonneville SSEi', 00' Sierra SLE, 1980 Goldwing GL1100 Interstate