Yes light dimming is common in car audio. Lights dim due to a sudden change in voltage. Whether that change is from 15.015v to 14.12v, or 14.5v to 11.84v. So the dimming alone doesn't necessarily mean you need more power. Check your voltage at idle and with the system at full tilt. If it's your headlights dimming you could switch to an HID system with a ballast, I don't believe voltage fluctuation will affect them. If it's your interior lights then I'm not sure what would be a good way to alleviate it. Moving a battery to the rear near the amp might help, but I'm not entirely sure.