Help with speaker wires

[Plunkett]

I've got a 72 chevy c10. The po installed a crappy radio/cassette player in the truck. My problem is that there was a speaker attatched to the kick panel on the truck but it wasn't hooked up. I bought some 6"x9" speakers to install under the seats. Which wires do I need to hook these speakers up? I assume it was the aftermarket harness for the radio and in that harness it has a gray, gray/black wire, purple, purple/black wire, and then some green wires coming off the harness that aren't hooked up to anything at all. Which wire do I need to hook these speakers up to?

[VH Coachworks]

What brand of head unit is installed? Do you know the model? Generally on aftermarket head units, the green-green/black, grey - grey/black, purple-purple/black, white-white/black are the speaker outputs from the radio. There should be 4 sets of speaker wires coming off the back of your radio, then find the rear channel sets and hook them up to your new speakers!

[Plunkett]

When finding the rear channel sets, do I just need to fade the radio to the rear speakers and just.check the.voltage on the wires?
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[tubbedII]

Typically the speakers are like this:

white/white-black = Left Front
grey/grey-black = Right Front
green/green-black = Left Rear
purple/purple-black = Right Rear
Black stripe being negative, solid being positive

I'd hook them up this way and then check the fader/balance to see if it's right and if not modify from there.

[VH Coachworks]

Either way will work. It would most likely be easiest to simply set the radios fader to the the rear. And then set the Balance to the left or right to determine which side is which. Then test each set of wires until you find the one and only set that are putting out voltage. Then set the balance to the opposite to find the other rear channel!

And Tubbed's post of the wire colors to what channel is also spot on to how most all aftermarket decks pigtails are wired!