why do some lights stop dimming?

In electronic theory 101 you learn that in most cases but not all electronic behavior can be predicted and modeled the same as water behavior. In the case of capacitance this holds true. Capacitance can be thought of as a reservoir in a water system and high voltage can be thought of as the highest point of a water source. Stay with me here because this will answer your questions. Typically the problem is that just like in water systems, a reservoir can not avoid a drought if the supply of water from higher altitudes (higher voltage sources) can not keep up. For example if you have a lake as your water source (battery under the hood) with only a small stream from a higher altitude is feeding it (small alternator) but it does not rain for a decade (your alternator can’t feed your battery fast enough) you will end up with a water shortage. Adding a cap to your system is like adding a pond down stream. It can only feed your needs for so long before it needs to be replenished. For that matter it needs to be filled by the higher water source to begin with (your battery and alt). Think of your alternator as rain Your battery as a lake Your power cable as the streams leading away from that lake Your cap as a pond that has the potential to help as a water reservoir And all of the electric devices in your ride are consuming this energy from various points within this system. When viewing things this way it easy to see that if your component is closer to the rain or lake or pond (cap) your component will have better access to the rain (energy). Problems arise when this system is not balanced correctly. Generally speaking people have had bad experiences with caps due to incorrect application. Most of the time too small of a cap is used for the job or the cap is inserted physically in the wrong place. In the case of dimming lights the pond needs to be physically close (as close as possible) to the demand (the lights if that’s what you are concerned about, the amp if that is what you are concerned about). In the cases where adding a cap relives dimming light syndrome, the cap happens to be electronically close enough to(at a low resistance to) the device of demand (lights), or the system is low enough in power and all other charging components are well enough equiped(alt, battery, cables, ground points, etc.) that only a small amount of voltage stabilization is required. I say low enough in power because most caps are only capable of very small amounts of voltage stabilization. BUT NOT ALL! But definitely the round toilet paper tube kind. Many people will argue that in a parallel circuit voltage is equal at all points. I preemptively respond to that very near sighted view, that in electronics 102 you learn about resistance in a parallel circuit. Read the second MECP book or go back to school! Yeah caps do have an effect. Will it positively affect your system? What are you expecting? No dimming lights? Better amplifier performance? Better tire wear? Some expectations are just unrealistic. Caps do have their place. If you add a cap to your system either expect nothing with the hopes for the most (it can’t hurt) or know more about your system. What type of power supplies do your amps have? How many amps is your alt? What kind of condition is your battery in? Is it designed to discharge and recharge at the rate your system demands? Is your power wire heavy enough? Are your grounds all up to par? Check under the hood! On and on and on………….You are plugging holes in a potentially sinking boat. Eventually you will find them all, but persistence is required.

Sounds to me like you have a pretty good grip on how this works. Just remember that large power wire is good for your amp, and that’s fine. Large power wire to your amp will not help your lights. Maybe 1/0 to your lights. just kidding! If your alternator is of reasonable size (in relation to your amp) and your battery is in good shape (and an appropriate design for your application) you won’t have dimming lights. Period! If you are running a power hungry SPL system get ready to start logically replacing components in your charging system until the problem is remedied. That’s how the game works. A good low resistance to ground is low. There is no real answer to that question. Ground your amps to a large steal structure that is welded to where the battery is grounded. Not to a piece that is bolted to a painted piece that is bolted to another piece. This might sound dumb but take a close look at the steel structure of your ride and try to find a single piece of steel that runs from front to back. Be aware of parts that are only tack welded to each other.   

you should get a bigger alternator with all of that current draw from your amps. thats 180A worth of fuses just for your amps. now that doesn't mean that they are pulling 180A all of the time and depending on the load on the amplifiers it could potentially be much less. the fuse rating is pretty much the fail point that the manufacturer doesn't want the amp to go past so under normal circumstances they wont pull that much current. still though your amps are probably maxing out your alternator which is why your lights will dim, its also not good for onboard computers and such. not to say a cap doesn't have its place, it is definitely not what you should be spending money on because it is not the root cause of your problem and an alternator would be money well spent and should solve your problems.