"Because if both speakers are "in phase" they will cancel each other out, correct?"

that depends on the frequency. while runinng the passengers mid out of mechanical phase will re-enforce some frequencies, it will still cause others to be cncelled out. the overall reseult is better though.

i have all kinds of reverse mechanical phasing in my car. it took a loooooong time to find the right combination.

the reason to reverse the mid is due to the position you are in when listening to them. if you were sitting in the perfect middle of the car there would be no reason to do so, but because you are closer to one side than the other, such actions may be necissary.