i am suprised you are still seeing a dimming with your hid, normally i reccomend a harness to tap directly of the battery for running HID so you have constant power versus using the stock wiring.

http://www.dxhid.com/purchasehidparts.htm

i purchased the one with the capacitor/resistor to take care of the light bulb warning along with flickering or strobing.

Try the big three upgrade and while you do that make sure you run your amp directly off the battery right now the issue you might be experiencing since it is not off the battery the fuse box might be regulating the amount of amperage going to the amplifiers and starving your system from extra power it can get running direct from the battery. When i use to install i would always tap of the battery direct and fuse it or use a circuit breaker.

http://www.darvex.net/Stinger-100-Am...source=froogle

i use a 100 amp circuit breaker a rockford fosgate but brand doesn't matter too much.

So to recap do the big three upgrade buy some screw on terminals as they are easy to work with,

http://cgi.ebay.com/PAIR-0-GAUGE-HEA...QQcmdZViewItem

http://cgi.ebay.com/PAIR-4-GAUGE-NEW...713.m153.l1262

http://cgi.ebay.com/PREMIUM-4-GAUGE-...713.m153.l1262
http://cgi.ebay.com/2000-WATT-AMP-KI...713.m153.l1262

you can just get a amp kit that is 4 agu or 2 agu and it has plenty of wire to do the big three upgrade and use that with the terminals so you have a secure connection. just hit up the guy on ebay and tell him what you want and he can quote you a better deal and maybe you can pay direct through paypal and save on fees.

after you do the upgrade, the look into getting the battery as you already said you need a replacement and try to buy the battery from a dealer on ebay who will cut you a deal and pickup a cap together so they give you a package price. also look at stinger batteries, or kinetik as they all make batteries and are about the same thing find a dealer that carries both so they give you a discount on the cap and battery.