is a power cap needed? well in most cases yes.

they are not just for cosmetic uses but help a great deal if you have a powerfull install or not.

symptoms without a power cap

head lights and interior lights dim with the beat of the music .your CD skips a beat or two, your amplifier may be trying to use more fast energy than the electrical system can supply and cut out at intervals then coming back on after a few seconds. Unfortunately, your car's electrical system is not thinking about the needs of your amplifier. Power is being robbed from your audio system by such trifling matters as: the engine and lights. This may leave your amplifier hungry for power, and worse, delivering destructive distortion when it fails to get it.
Understand of course, that no capacitor is a substitute for an adequate amplifier to provide the amount of bass energy you want to project. Nor will it substitute for an inadequate car power supply. You may have to install a stronger alternator if you have one of the more powerful systems. The rest of your system must also be "up to spec" to serve that purpose.

so how do they help.

power caps are energy reservoirs that store the necessary power your amplifier will need to punch those big bass notes.they reduce headlight flicker upto 90% sometimes getting rid of the flicker all together.interior lights also stop dimming also so does the head unit (i must state not all head units do get this problem but most do) a more smoother bass sound and a lot less load on the amp stopping amp cut out as energy is taken from the power cap itself. giving power where it's need most the amp! 

best place to install a power cap

Mount them as close to the amp as possible to read the values and use at least .5 farad (500,000 microfarads) per 500 watts RMS of power output.

THIS INFO IS TO MY KNOWLAGE IS CORRECT IF NOT SURE PLEASE CONTACT AN AA STAFF MEMBER.