I read, that some cars (like mine) have a "slow power regulator".
For example, if you lights dimm when a BOOM! hits the subwoofer, it is not necessarily because of lack of amps... You can try this by having a continuous BOOOOOOOOOOOOOOOOOOOOOOOOOOM (low note) for a long period of time (at least 5 seconds). and if your lights dimm, and dont come back to normal, then there'a weak current, if after half second it comes back to normal then, it's a slow power regulator.
In that SPECIFIC case is necessary a Capacitor? (just to avoid the dimming, but knowing already that we have enough power).