I finally got to testing this out.

Resistance showed me the ground. Interestingly, when I turn the fader all the way in one direction, two audio lines decrease resistance like the ground wire. So now I know which are front and back.

Unfortunately, "AC V" and "DC A" settings on my unpowered analog voltmeter didn't show me anything coming out of the stock radio. With volume high or low, balance and fader centered or to one side, I saw nothing. What type of signal is line level output?

I guess I'll try hooking up headphones to the stock radio to find left and right. Or hook up an iPod to the stock amp.