Hi Everybody,
I've been reading this thread for a while now and have to say I'm tickled that everyone is finding out about this. Since I've designed amplifiers and speaker systems for 25 years now from (concept to release), and since they can be a little complicated, I would like to throw out some real information.

I know some of you already know this, but for a few that don't know yet, the capacitors everyone is using are Electrolytic capacitors. Electrolytic capacitors are polarized.

Some of you have raised very good questions, like polarized or not. The fact is, if the amplifier output ever produced polarized voltage it would immediately destroy the speaker, so it doesn't. The first amplifiers made (dubbed class A amplifiers), had dc voltage on their outputs. These amps used electrolytic capacitors on their outputs to block the dc voltage from reaching the speaker. So I wouldn't worry about this issue.

Capacitors have a voltage rating. Well, the voltage in our CFs are limited to 12 volts so I would use something in the 25 volt range. That gives a little buffering and will make the capacitor last longer.
Capacitors also have a current rating, referred to as ripple current. This rating is what is used to make sure the power in the capacitor is not too high and doesn't cook the capacitor. I would use a capacitor with as high a current rating as possible, though in the audio range this isn't significant using modern electrolytic caps.

When an electrolytic capacitor fails it either shorts itself out, or it opens up like a light switch. So if you notice your bass returning, this has probably shorted and is acting like a closed switch. At any rate, it's a safe failure mode. If the cap fails open instead, the speaker will just stop working and still, a safe failure mode.

Putting the cap in the negative wire will, in most cases, be safer if the wire comes loose and contacts the body. But, in the case of our CF amps, neither side is ground. The amp uses a common method called BI-AMPING, which drives one side of the speaker negative while driving the other side positive. Hopefully, the amp is protected from short circuits (probably is, actually).

Have I done the mod? No. I replaced the head unit so that I could add MP3 capability and an MP3 compatible disk changer. I do turn down the bass on the new head unit though, so I'm definitely going to install this mod. Turning down the bass to reduce the distortion in the door speakers also reduces he bass in the subwoofers (it's a high pass thing, same as everyone has been doing with the caps in the door speakers). I too like clean, punchy bass.
I've discovered that punchy bass requires some midrange action. So, by reducing too much bass in the door speakers, one might reduce punchiness (It's a harmonic thing).

Well, this is pretty long winded, but hopefully entertaining for someone.

Best Wishes,
Polywave