A capacitor is only good so far as the audio system isn't trying to pull too much from the electrical system. A cap doesn't provide more power, it's designed to 'stiffen' the voltage to the amp, nothing else. If the current isn't there, a cap won't help. The alternator must have at least 20% more amperage power than the entire vehicle and sound system combined for a cap to be of benefit which is ironically the same requirements for an amp to be efficient.
so therefor, a capacitor will only be a waste of money for you, considering it wont help you at all, considering light dimming isnt what they are used for
and before you do a new alternator upgrade, you should do the big three upgrade
also if your going to upgrade alternator, you should upgrade to an optima yellow top battery