Coming from a Audio Visual installation back-ground, I can tell you that the polarity of speaker wiring does not matter (unless its a 70v/100v line system, which car audio isn't)...with one important condition. Both (or all 4 speakers) are wired the same way. This is because you want all the speakers moving in the same direction at the same time...this is called in phase. Think of a speaker as an electric motor...or 2 motors in your case...you want them all "spinning" in the same direction. If you wire it out of phase, it wont damage anything...but you will notice a perceived loss of power (loss of bass to the untrained ear), this is because the sound waves produced by the 2 speakers are having to compete with each-other to move the air where as they would normally work as a team moving air in pulses at the same time.