A capacitor, which stores electrical power to deliver on demand, could help. A cap cannot correct a battery or alternator problem, but can alleviate some of the intense current draw associated with brief low frequency musical peaks (like hard bass hits).

Typically, the main reason that you would want to add a capacitor to your audio system would be to gain better sound quality, especially from the low end of your system. Low frequencies require more power in order to be reproduced at the same level as higher frequencies. The capacitor improves the sound quality by storing electricity from the battery (which is provided by the alternator), and releasing it very quickly when the amplifiers need it. The capacitor then recharges from the battery. As you can see, the capacitor helps alleviate some of the direct stress that would normally be on the battery and alternator, but if there is a power problem in the vehicle it's best to go right to the original source and upgrade the alternator

If your lights dim either when the bass hits or when multiple accessories are turned on, then the best thing that you can do to fix the problem is upgrade the alternator. In an automotive electrical system, the alternator provides all of the electricity that the vehicle uses. The battery is maintained by the alternator, and stores enough electricity to start the car and run the accessories for a short time while the engine is off. If a stereo system that has a high amount of current draw is added to a vehicle, this can possibly be too much of a load on the factory alternator, especially if the load is near or over the alternator's maximum output capabilities.