Squirrel is correct. and it's funny you posting the battery test as I just posted that to someone else who is going nuts trying to find hs speaker wires. small world.
Anyhow, do what he says.......Phasing as to do with the time/distance that the sound from stereo speakers reach the listener. Basically if you took 2 house speakers and sat directly in front of each, and they are electrically in phase (more on that soon) they would also be acustically in phase. Now if you stayed seated in the same position, and had some else start moving one of the speakers away from you, you would start to notice a dimishing of bass response. The sound energy, (afterall a speaker is really an air pump in a sense) is now not equal so they are not working together to sum thier total energy.
Now has the one speaker keeps moving away this can be measured in degrees. At a certain distance the speakers can be considered as 180 degrees acoustcally out of phase. At this point you will have the weakest bass response. If this seems a little confusing don't worry, it's also been so long since I was teaching this I am actually struggling with terminology but can still make the point.
Now in a car you are basically stuck with where the speakers will be placed, yes you can modify things and put them almost anywhere but they will be fixed so as far as acoustcal phasing there is not much we can do in a car environment. You might have seen so alla out demo vehicles where they put the drivers seat in the middle and set it way back. Well this not only benifits acoustic phasing, but staging and imaging at that level in a car can perform almost as well as a home system. But thats an extreme example.
electricalphasing in a car, which is the real issue here there is only 2 ways you can have it wired. either in phase (0 degrees or 360 degrees, don't matter, same thing) OR 180 degress out of phase. For this we will only discuss 2 front speakers but all the speakers should be in phase. Sometimes sub amps or electronic cross overs have a phasing switch for the woofer(s).
now although the woofers themselves if using more then one must be electrically in phase with each other to produce soild bass, the woofers themselves can be out of phase with the rest of the speakers in the car since like in my earler wxample, distance has an effect and where the enclosure is located, the size of the car, etc, will effect the distance and what part of the bass wave hits your ears and when.
All you need to worry about with your 2 front speakers is that they are wired in phase electrically (like squirrel says, the the positve terminal of the left side speaker is going to the positive side of the correct output of the deck/amp and the same for the right side. Too many times this is overlooked either because the installer doen't know the color code, or was careless and just made a boo-boo. Thats why I say to always double check when there a bass response problem no matter who. Mistakes happen. don't forget the rears also. Just rinse and repeat the battery test on those as well.
when the speakers in your vehicle are electrically wired 180 degress out of phase (one side has the positive and negative reversed in respect to the other) the speakers move air against each other instead of with each other, effectivly cancelling most of the bass, mid bass out, which sux.
Now keep in mind if both of the speakers were wired backwards it would make no difference that you could notice by ear. Both speakers are still electrically in phase. audio is AC, so the speaker is just moving back and forth moving air. Now even at the lowest bass note a human can theoretically hear, 30hz...that cone is moving back and forth 30 times a second. If anyone could notice that by ear I also have a space ship in my garage and it can time travel.
I know this reply is long but I actually explained this the simplist way, over simplified it but I think you get the jist.
Easy one guys.