I'm trying to integrate aux input to the stock amp on my sister's 1993 Volvo 240 wagon. The amp takes a 6 pin "DIN" input plug from the radio. Some Volvo forums unclearly report that the pins represent the positive wire of four speakers, a shared negative wire for the speakers, and remote amp turn on.

Any idea how I can determine which wires are which, or if this estimate is even correct? I tried a voltmeter to the radio's 6 pin output, assuming that adjusting the fade/balance to play through one speaker at a time would show signal only on that line. Unfortunately not so.

I grounded the voltmeter to the power/control harness ground and put the positive into each of the six pins, only getting signal on the center pin (supposedly the shared ground). Putting the voltmeter ground in the center pin and the positive on the others showed nothing.

Or could I used my hacked 1/8"-stereo-to-speaker-wire adapter to send input to the amp? Could I hurt anything by sending audio down the ground, amp sense line, or other weirdness?

No, I'm not integrating an empeg alongside the stock radio, sadly. I'm going to put a source switcher in for iPod aux in. I think it'll be a cool stealth install for my sister who doesn't care about audio quality; only wants to keep her brick totally stock.