Do I need a capacitor?

I'm running two MTX 7500's strapped by a kenwood KAC-9103D 1800w monoblock amplifier. that is 900 true RMS watts running all together. I have a 1996 chevrolet cavalier with a fully system including DVD 7" LCD screen in dash, and 4 polk audio speakers (two 3.5" and two 6X9) all together running around 1000w of TRUE rms wattage. Do I need a capacitor and how big or farads should it be? Thanks a lot.