Ok, I would still suggest trying a Optima battery(its deep cycle and starting battery): http://www.optimabatteries.com/publi...eep_cycle.html

A deep cylce battery is better at supplying current over a longer period of time. The type of battery found in your car right now is a "starting" battery and is only designed to supply a short burst of current(when the starter needs it) so this is most likely why your lights dim. Once the car is running, the battery acts like a sort of capacitor once its charged cuz the alternator supplies the current at this point. By replacing the battery will be able to smooth out the dimming because a deep cycle has the abililty to supply current a bit longer than your current battery. On the other hand, a capacitor will act like another battery and supply a short burst of energy when needed. Its your call, I think Optima batteries run about 150 and good caps run about the same. I personally would go with the battery in your case and if this does not help, look into a cap. I think a alternator would be a bit extreme at this point.

Also, if you ever get chance, try replacing your 8 gauge wires with one 4 gauge and a distribution block in the trunk. It will be a cleaner setup and make sure you have good grounds to both amps.