Who knows how to check speaker wire polarity?

oldmattb wrote:
So we are talking about two speakers, and not the full surround array. Hook them up and listen. Reverse the wires on one speaker and listen again. Choose the best of the two.

If you can not get access to the connector on the radio, this is about the only thing that will work. Audio going to the speakers is an AC voltage so trying to measure DC will not work. The connections are marked plus and minus just for reference for hooking up the speakers. All that matters is that both speakers are hooked up the same. If the speakers are wired incorrectly one speaker will tend to cancel out the other since the sound is out of phase. It's best to sit in a spot that is centered between the speakers.