The thing is, people get confused in the real purpose of a cap. It is not used to cure or prevent dimming lights nor will it change the way the system sounds (both are misconceptions).
To understand the purpose you need to understand how they work.
A cap does it's best to keep the voltage at a certain level, but if the voltage changes so will the cap voltage. The cap just makes the change smoother. The diming light syndrome isn't a voltage issue but rather a current (amperage) drop. The voltage simply follows what the current does. A cap does not help this situation at all. A cap doesn't provide more power nor does it try to correct it.
A cap is perfect for a noisy voltage line or if the car has fast, erratic voltage spikes/drops. This is why caps are used in AC to DC power supplies as they are used to smooth out the DC ripple for a more smooth DC output.
The ONLY cure for dimming lights is a more hefty amperage source, the alternator.
The REAL powerplant of the car is the alternator. The ONLY purpose of the battery is to start the car and run the accessories with the engine off, nothing else.
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To install:
It's best to install it as close to the amp as possible. You may need to get additional power and ground wire the same size you have now. Otherwise, cut the ground and power wire leaving enough room to mount it. The overall length of the ground wire should as short as possible (18" or less if possible, not critical).
Remove the fuse at the battery. If you don't have one, you need one that is rated the same as the total fuses that are on the amp. It must be no more than 18" from the battery.
Connect the ground from the chassis to the cap then to the amp.
Connect the the power wire (from fuse by the battery) to the cap then to the amp.
To charge the cap, get a 12v light bulb with leads. Temporarily connect the power wire from the battery to one side of the bulb (at the fuse) and connect the other side of the bulb to the wire going to the cap. The bulb will light and over a period of time (10-20 seconds) it will get dimmer. When the bulb goes out, the cap is charged. Basically, your going to put the light bulb where the fuse was.
Now you can re-install the fuse.
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IMPORTANT: Make/print a sign/note that states:
"WARNING: This vehicle employs a large electrolytic capacitor for the electrical/audio system. The capacitor holds a charge EVEN when disconnected. When changing the battery, remove this fuse. Take caution and charge the capacitor prior to replacing this fuse. SEVERE INJURY OR DAMAGE MAY RESULT IN FAILURE TO COMPLY!"
Have a big red arrow pointing to the fuse and one of those yellow triangles with the exclamation point in the center. Laminate this sign and place it near the fuse by the battery.
Something like this:
http://i81.photobucket.com/albums/j230/sparky3489/CapWarning.jpg
