regardless of this situation, a capicator is a bandaid to an existing electrical deficancy when related to dimming headlamps. obviously car manufacturers dont install an alternator in a vehicle with more than are20% (or so) percent 0ver t0lerance. so his stock alt supplies what? you said 80 ampres? so his car in theory should use 50~70 ampres? even with the capicator, the electrical system is being overdrawn, and the alternators electrical induction is forced at maximum. so the windings on each side of the generator are warming up rapidly and will most likely end up shorting out or burninng up the regulation circuit.
the
problem is NOT the dimming, but that is what is noticeable. the
PROBLEM is that one of these days your car wont run becacuse the charging system is going to fail and the owner spent the money on a bandaid rather than a solution.
so you installed one of these? wow. steep price!!
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=140051430351&ssPageName=MERCOSI_VI_ROSI_PR4_PCN_BIX_Stores&refitem=140050854309&itemcount=4&refwidgetloc=closed_view_item&refwidgettype=osi_widget
why not go with this for a few more dorras? (assuming you buy one for his make\model)
http://cgi.ebay.com/ebaymotors/200-amp-Iraggi-Alternator-99-2000-Jeep_W0QQcmdZViewItemQQcategoryZ33573QQihZ019QQitemZ7955200588
so in recap-
capicators will fix dimming sometimes. yes. is dimming the real problem? no, no its not. dimming is an indicator that should make you realize there isnt enough recharging power in your electrial system to run the car, stereo and headlights at the same time.
BUT - capicators are useful in some situations, such as the voltage regulation situation. they do have a good amount of capatance and will steady the input voltage from the alternator wich is a good thing, but IMO thats all they are good for.
and on a second note - regardless of weather or not your headunit has 3v, 8v, or 24v preouts, that is the MAXIMUM output voltage. we all know that audio consists of AC frequency cycles, so maybe, just maybe, a cycle in the track will reach maximum amplitude and the volume on the headunit will be at max, and it MIGHT output 8v for a milisecond. so a "true 8v" means it CAN, not that it will be there at a constant 100% of the time. get a voltomitor and set it in series with your RCA's and drive around for a week. tell me if it ever reached 6v and ill be suprised.
and that is my 2 cents.