You don't need a cap. A cap stores power, yes. But it gets that power from the alternator/battery. A cap will help keep the voltage more constant in the car, esp when using big amps and loud bass hits. It helps with dimming lights also. Def check your alternator, it is THE PRIMARY source of power in your car. All power comes from it, when the car is running and charging. Go to autozone or wherever to have your alt and battery checked out for free. If your battery is somewhat old, I would get a new one, to be on the safe side. They really aren't that much, and is better than being stranded somewhere you don't want to be stranded at.

13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
My webpage: http://eclipsed4evr.home.comcast.net
-1998 Mitsubishi Eclipse RS-T- "Toy"
-1992 Plymouth Laser Turbo AWD(SOLD)
-2000 Honda CR-V(daily)