sounds to me like a wiring issue. I've run same size setups w/ 4 gauge and no dim. i think you might have your cap in line wrong. Also, i've found that distribution blocks make a huge difference. I use 4 fuse dist blocks for hot and cold side....ALWAYS match your wires, and keep as short as possible.

 

Cool. My distro block is not fused. It's just a 4g input and lets you split it into 4 8g outputs. I pretty much have the same as picture.

 

2 questions. is the car running when this happens? If so, what is it the car idle at? if you are idling at 500 rpm like mine does, your alternator is hardly doing ANY work. Your lights will dim if your fan kicks on. this is normal fyi on a car thats even a few years old. (maybe new too) in this case, just turn off your headlights while parked and save some $ The only time that i would be concerned is if you are driving and they dim. but as i said before, => setup in the past and it was fine while driving, and i REALLY had to crank it to even get dim at idle w/ 4 gauge wiring, and only a 1 farad cap.

 

blocks like this http://www.tune-town.com/yho/4fblk.jpg have ports on the side of the hot end for a cap

edit: just googled for a cheap one for ya. i used some over priced titanium ones if i remember correctly

 

It seems like most of you are forgetting that a battery will not supply enough current to power the amplifier for a very long time.... if you want to run that kind of power you need to get a better alternator, plain and simple. otherwise, you could cut your power in half and not notice any difference in your sound and fix the problem (you may feel a SLIGHT difference but you will not hear a difference).