Adding a cap to the system may help your dimming but it's not going to help all the other electronics you're stressing. (A cap will also add even more strain to the alt, keep in mind) You'll probably end up paying more replacing / repairing blown amps / electronics than if you just did it right the first time and upgraded your alt. Always do electrical before adding amps to a stock system. Your factory alt isn't meant for multiple-kilowatt stereos. The most it's ever meant to see is car running + AC and headlights.