What kind of amp and how many watts, the cap may do more harm then good if your going to be drawing a ton of current on a stock electrical system. A cap only works to keep small dips out of the voltage when the current in the car is higher then the draw of the amp, if the draw of the amp is higher then the stock alt output then your cap will discharge on some notes as your alt kicks out from strain and keep your voltage down even longer due to the fact the cap now has to recharge on the next lull in the bass to try and play catch-up.