Capacitors are completely useless in car audio.
They do not take any load off of anything. Your alternator charges the capacitor, so maybe for a second or so that alternator isn't directly powering your system, but once that capacitor drops a couple volts, which happens almost instantly, the load goes right back to your alternator, and if the alternator isn't powerful enough to keep up at that point, the load jumps to your battery. Now what good did that capacitor do? Oh right....none...
Your alternator and battery ultimately carry the ENTIRE load of the audio system.
Follow me here and tell me if this logic makes sense....
-Alternator powers capacitor
-Capacitor powers audio system
=
-Alternator doesn't power audio system
Is it just me or is that far from a reasonable conclusion given the previous two points?
Some people say "well the capacitor is just supposed to help your alternator keep up with the power demands blah blah blah" in essence they are saying that the capacitor makes the alternator have a smaller power demand because of the power it provides temporarily. This again just doens't make sense logically. Every single bit of power the capacitor has, came from the alternator, and the capacitor will never have a single bit of power in it again after it has drained unless the alternator powers it up.
Has this sunk in yet? You don't need a capacitor, it won't help a weak electrical system, it doesn't save anything from powering anything.