Capacitors are really not that effective for helping with a constant drain on the system, they do help maintain a constant voltage at the amp for short bursts but with high draw situations the cap becomes an effective paperweight. Upgrade the factory ground, the engine ground, then the battery. The reason your electrical is dying out is that the typical lead acid battery does not handle vibration well, and with a set of good subs in the trunk you can kill one in pretty short order. Upgrade to a gel cell battery like the optima or for the ultimate in vibration resistance a dry cell battery (the dry cell is very costly though). Once the new battery is installed have the system load tested to see if the alternator is working well, with a bad battery you can also burn up an alternator pretty quickly. In case your wondering I am a very well qualified audio guy and have well over 15 years of automotive mechanical experience so I know what I am talking about here.
