If you are just running factory size speaker (6.5" front doors, 6x9" back deck) you won't need a cap. Caps are kind of overrated, IMO anyway. Cars made in the last couple of years have pretty high current alternators (at least 90 amps) to operate all the power windows and high current stuff as well as the fuel injection and ECU. Cars are tested under full load when designed.... that is A/C on recirc, full fan speed, all interior, trunk, and exterior lights on, cruise on, windows, door locks, and moonroof motors running, radio on full blast, wipers on high, you get the picture. If the alternator can pass the full load test, it will work fine on your amp. Those caps are really for competition, to show off, or for an older car with a weak alternator.