From AMG
CAPS: Do you need one? Very subjective subject. Basically a cap is like a reserve of electrical energy, "almost like a battery" On some systems when a deep bass note is played, the battery can not delivery enough electrical energy to the amp, this cause the bass to be delivered with less than its optimum ability. What the cap does is act as a reserve supply that can be called upon as and when needed.
A battery can not respond quick enough to the instant demands some bass system put on them and this is where the cap helps out because it can respond and recharge much faster than a battery.
There are some people who claim that a cap also helps in clearing up some of the middle and upper frequencies in a multi amp set up.
Before I went over to Digital class D amps I had a 5ch Genesis amp and their big Monobloc amp running my system [www.efmax.c.uk/serv01.htm]I had one Optima battery upfront and a second audio battery for each amp. BUT no cap. Considering this system could drop some REAL bass and that, that Monobloc at full tilt could draw more current than my alternater could put out, there was still no light dimming or degradation in bass output.
The key here is the power supply. Good and correct cable, I used two runs of 4AWG cable, fused at each end, to the batteries. Decent alternator and the consideration of a second battery.
I am not a great lover of caps but they do help some people out. It is worth noting that the better the amp the less need there is for caps. Often I see caps on systems that have very good branded amps, but their owners have been seduced by marketing bullshit.
As for fitting them. Safety is a real key issue here. Done wrong and this things can seriously harm both you and your car.
They should be mounted as close to the amps as possible. They should be earthed well and if possible to the same point as the earth on the amp/s. You need to take the power supply from the battery to the +ve side of the cap, then take a power feed from here to the +ve input on your amp.
SAFETY.. can't stress it enough, these things if shorted out can torch your car as if you set fire to it and poured petrol over it.. SAFETY...
Hope this helps... EF MAX