There's so much misunderstanding about caps for use in car audio.....whether they are needed, "do I need a cap?", "caps don't work", "get a battery instead", "do the big 3", "buy an alternator", "I have a cap and my lights still dim"...etc
First, don't get a cap to fix dimming lights.....although they sometimes can fix this. Caps aren't a battery, they aren't an alternator and they don't create any voltage, nor are they a replacement for "the big 3" .........they work in conjunction with any and all of these and act as an energy storage buffer to instantly(faster than any battery ever could) supply this energy to your amps during large transients. If your system constantly exceeds this energy buffer then you need to beef up the weak link in the chain elsewhere in your electrical system.
Not all caps are created equal.......stay with the 1 farad canister types and your usually pretty safe, the rule of thumb is 1 farad for every 1k watts. What makes a cap desirable for car audio is it's ultra-low ESR(resistance). An ESR of .001 is ideal, .002 is good and .003 is borderline.....above that and a cap is useless. The large "digital" and carbon/hybrid multi-farad units do not work because their ESR is too high.......sure, they store lots of energy but can't discharge quick enough to do any good.....any battery would be better than one of these units. Don't get sucked into the marketing hype of batteries that act as capacitors either.....they still don't discharge quick enough to do what a low ESR cap can. Be careful of digital readouts and large distribution blocks on caps too, sure they are cool and very pretty but add resistance....I always remove mine.....low resistance is what caps are all about.
And to the OP, we need more info about your system before either could be suggested.