Quote Originally Posted by Joe Isuzu
...that possibly the polarity is reversed at the speaker?
That would be the most likely spot where a reversal could occur, especially since you've used adapters to tap into the factory plugs. Although it's been awhile since I've done any stereo mods, most speaker wire I ever saw was either color coded (bronze and silver for example), or actually marked (+) and (-) on the insulation.

It would make sense that aftermarket speakers would have at least one terminal marked so that polarity was easily established, but I don't remember that always being the case, hence the trick of using the 9-volt battery to make sure.

The problem occurs when one side (say the back right) is connected one way, but the other side (back left) is connected just the opposite, making the speakers fire 180 degrees out of phase, and in effect, canceling each other out. (Not the most accurate technical description, but you get the idea).

It sounds like you just added rear aftermarket speakers? If the speaker wire from the adapters is marked (+) and (-), you could still use the battery technique to get the polarities correct and make sure each side is hooked up the same way.