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If you knoow where the factory amplifier is located, you may have to use an amplified computer speaker to find the input wires.  This is an easy process.  You will need 2 small alligator clips and some wire other than the amplified computer speakers.  If you are willing to try this, I will instruct you on this process.

With the computer speakers powered up and volume set to half, radio powered up and volume set to the same level.  Ground the wire as instructed in the diagram,  Now touch the wire that you have connected to the end of the connector, you should hear a faint sound in one of the computer speakers.  Once you confirm that you can hear when you touch your finger to the wire, you are ready to start probing wires within the plug.,  Touch the end of that wire to each wire in the plug.  You should hear audio on 4 or maybe 8 different wires.  Once you find these wires.  Fade and balance the radio to only one speaker.  Probe the wires again to see which wire corresponds to the channel you have playing.  Balance or fade to a different speaker and probe again.

Let me know if you have signal on 4 or 8 wires.   Also if you have a multimeter, set it to AC volts and turn the volume all the way up on the factory radio.  Amp disconnected of course.  Ground the black meter lead to the chassis, touch the red meter lead to one of the wires you just found, what is the highest readiong you get on the meter?  Leave it on the wire for 20 seconds or so.