If you have lights dimming, the first thing I'd check is the gain on your amp. Make sure it's set properly. I don't recall what gauge wire you are running to the amp/carpc/everything else, but it might be undersized for the amount of power you are drawing (forgive me for not reviewing the thread, I'm off to bed in a minute). You should run at least 4GA from the battery to the amps/distro block/whatever (IMHO). You should also get the battery checked, because a low battery can contribute to flickering and other sudden power loss, and also get the alternator checked.
I agree with soundman - don't waste your money on a cap. It won't help an underpowered/overtaxed system.