High 12's is maintaining/charging the battery. .1-.2 volts is normal as it's 15-17 feet of wire to amp and it comes from the battery not the alternator.
If the dimming really bothers you(it isn't hurting anything as long as you're not sitting listening for extended periods) then you'll need a fast acting, adjustable regulator without a doubt and possibly an alternator. Keep voltage under 14.5 because it'll cook the battery. I wouldn't mess with it myself, But remember HID lights don't dim.
My interior lights dim as well lol.
After talking to some other people, I think I'm going to go the route of a cap instead of the alternator. Everyone was mentioning the same about the alternator still charging and somewhat keeping up, and the general thought is that the cap will fill the void where the heavy bass hits are and should help eliminate the dimming of the lights. Plus its a lot cheaper than going with a new alternator. I still consider it something of a band-aid, but hopefully it works out.
