Your stereo is most likely to big for your stock alt to handle. look into a bigger HO alternator from someone like Irraggi Alternator or DC Power. then from there you may want to get a large dry cell battery like an Optima or XS Power.
First you should upgrade your Big 3.
1) Battery negative to chassis
2) Alternator to battery positive
3) Chassis to engine
that will help with
1) Reduced dimming and smaller voltage drops
2) More stable voltage and better current flow
3) Less strain on your vehicle's charging system
buy some 1/0 wire and some terminals from Knukoncepts.com to get that done. doing the Big 3 upgrade some times will eliminate dimming but for some systems that is not enough by itself.