The capacitor is not a "backup". It can supply high currents for a fraction of a second (there is a formula to calculate the size according to the current and time) and avoid clipping. But a good 12 VDC connection can supply that current, too. So it depends on how loud you really want the sound (in watts) to determine if you need a large capacitor.The kind grind. I love the use of the bottom deck as a reflector--back in silently, pop the hatch, and let the party begin!
What kind of power are you driving on the amp? Did you need to back up the amp with a capacitor?