Why don't y'all try recommending less expensive fixes first - i.e. remove the rear deck speakers.
And if he already bought a cap, he might as well keep it. They're not nearly as bad as many of you make them out to be, and they are certainly not detrimental.
if you play your music loud for any length of time, yes the become a problem.
they can only supply current in an instant burst and not over a length of time.
after the first loud bass hit, it just becomes a draw on the electrical system and will not help anything untill its is allowed to charge it self back up.
all a cap will do is just pass current during long, loud sessions with out boosting or adding any extra current.
just doing the big 3 will give 3x the instant current and allow more current over a longer time than any cap could ever come close to doing.
adding a small battery, say like a stinger spv35 or the likes would be the better idea and that battery cost about as much as a name brand cap.
the cheapest would be to loose the cap and run the big 3 now.
in my system, iam running a stock 100a alt, 2 red top batteries and have the big 3 using just 4ga wire and i have near 0 light dimming and my voltage NEVER drops below 13.2v. this is running a fusion nvam250001 at .5 ohms and a fosgate 500s4ch bridged to 2ch and running at 2 ohms. its not the biggest and loudest but it can and dose draw a shit load of current but i have no issues.