Skip it. They are a total waste, they do not stop dimming headlights, they do not change the sound. Dont waste the money.
Upgrade your big three wires as shown here, this will give you more benefits than any capacitor will! Caps are for newbs who dont know any better.... and it costs next to nothing.
http://www.youtube.com/watch?v=p82xRzCr-lc&feature=player_embedded
http://forum.sounddomain.com/ubbthreads.php/ubb/showflat/Number/312025/page/1
http://forum.sounddomain.com/ubbthreads.php/ubb/showflat/Number/312025/page/1
It doesnt matter what countless newbs say, you dont NEED a cap, it will NOT "save your battery and alternator". Its a complete myth.