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I am one to belive that a cap is nothing but bad news to your cars electrical charging system. All the things said before are true, it does help dimish the power spikes when your at a stop light, or like situations. With out a resistor to limit the amount of current drawn then it will essentially be depleted after a long red light and will be getting charged, and uncharged, to feed the amp at the same time. Thus causing a larger drain on your electrical system. Of course this is just my opinion as to why I will never have a cap in my system.
What I would/ have done in my cars is:
1. Get a new/better battery. Optima red or yellow tops are usually preferred, depending on your desired result
2. Upgrade 'The Big 3'. This includes increasing the size of the wiring A. between the battery(+) and the alternator(+) B. between the battery(-) to the body or frame and C. the chassis to the engine block
3. Do all of the above with the larger then stock wiring.
More on upgrading the big three can be found
here.
have fun