Hey ken thanks again for the help I Have power directly to battery its usually at peak bass hits. As for the current amp rating i'm not sure i have one mone 1100w memphis amp powering two kicker L7's
Your 1200W mono amplifier will probably draw over 100amps at peak. This is your problem.
A fully charged battery will put put out something like 12.7V unloaded and something 12V or less under a heavy load. The alternator is regulated to put out somewhere around 13.5-15V on most cars. The higher voltage is needed to push enough charging current into the battery. When you put a heavy load on the battery, the voltage will drop down to 12V or less as the load gets close to what the alternator is putting out. The dimming you see is a fluctuation between this 14V or so volts of the alternator and the 12V of the battery.
I could not find an output vs RPM curve for the Denso alternator in the Nitro, but here is an example of a curve for a heavy duty alternator on a Ford V10.
http://www.americanarmature.com/images/APSweb/AAI Nitro II G Engine Speed.pdf
Notice how max output is alternator is somewhere around 2,000 RPM. at 400RPM (idle speed) this 250amp alternator is putting out less than 50amps.
If we assume that the alternator in the Nitro can put out a maximum of 160amps, at idle it may only be putting out 40-50 amps. This is fine for lights, ignition and maybe the stock radio, but throw an extra 100amps at this and it is no surprise that you are seeing light dimming.
Here is where a big capacitor can help. If we assume your amp draws only 20amps for the bulk of the music, but 100amps on a bass beat, the dimming may last less than a second or so. A big capacitor 1-3Farads, can store power during the low power time and dump it out for the bass beats. This does 2 things for you, 1. May reduce light dimming on bass beats. 2. Will reduce clipping leading to better bass sound and help protect your speakers from square waves.
You should connect the capacitor as close to the amp as possible and use very heavy wire.
If you don't have a voltmeter, go out and buy one, this will help you fine tune your DC power system.