I USE TO HAVE A REGULAR BOX WITH TWO 12 BUT I DONT UNDERSTAND WHEN I SEE SPEAKERS IN THE TRUNK REVERSED NOT IN A BOX HOW DOES THAT WORK AND DOES IT HIT THE SAME OR WHAT?

 

Yes it hits the same, its call " Inverted ". Only think diffrent you will need to reverse the polarity if you invert the subwoofers... PS stop using CAPS

 

does the space in the trunk have a factor like a enclosed box has on the speaker or does it not matter

 

I think he might be refering to an IB installation. Although I have seen these infinate baffle type installs work, I dont think the Mustang would be a good candidate for it. The rear deck has vents in it that would keep you from having your trunk sealed off from the cabin. You could take the time to Fiberglass your rear deck to eliminate the vent, but that is still less effective than just building a box for your trunk.
Of course you all are welcome to correct me if I am wrong.

 

The only reason to do this is to match amplifier Impedence. It allows for more powerhandling, but in no way does it give your more SPL then running the 2 independantly the traditional way.