i am wanting to build a fiberglass subbox between the strut tower and seats. i have been just using an old box in the back and it sounds great then i moved it to where i want to build the new box and it doesnt hit near as hard. i am guessing it has something to do with how the sound bounces off the car in the different places. if i ported the box to the rear of the car would i still get the same sound as i would if the sub was back there?

 

Sub-woofer location is not critical. Frequencies below 100 Hz are non-directional. Meaning that the ear does not try to find the location of the source.

That said, one primary problem is standing waves. A standing wave is a sound wave from the woofer fighting a reflected sound wave. This is a "dead" area and can sound horrible distorted. In a square room you can intelligently predict the location of standing waves, and adjust speaker location accordingly. In a vehicle with multiple reflective surfaces this is more complicated.

I recommend that you leave the speaker "free," and experiment with different locations. Choose what gives you the best result.