Here's how I recommend wiring up the components in your car.
Hook up the positive to positive, negative to negative on every speaker in the car, except the front passenger woofer. On that woofer, hook up the positive to negative and negative to positive. (In essence, wire the woofer up wrong). Keep the tweeter wired up "correctly".
With the woofers wired up in-phase ("correctly" for easier terms), the sound waves are firing "into" each other. When this happens, some of the frequencies get canceled, thus resulting in lowering the sound volume of that particular frequency.
When you wire the passenger woofer "incorrectly" the sound waves are firing in the same direction, which will not cancel them out. It will amplify them slightly, but most people will not hear the amplification.
Hope this helps! Feel free to email me and drop me your IM. I'll be glad to help out in any way I can!
