In car audio, phasing refers to sound waves, and the time they reach the listener's ears. When the speakers are setup in phase, it means that the signal would reach your ears at the exact same time, BUT, this is only if you are sitting in the direct center...obviously in a car this isnt gonna happen.
The idea of wiring out of phase is so that because one set of speakers is farther away from your ears than the other, the signals will arive to you IN PHASE, thus creating a more realistic sound stage.
I reccomend trying it. Listen to your radio how it is now, then wire either side out of phase (switch the pos and neg wires at the amp)More than likely you will notice a difference.