It really is imperative to use non-polarized capacitors. The really short answer why is that since we are filtering an AC signal from an audio amplifier, the capacitor will see both positive and negative polarities. A polarized capacitor cannot survive a reverse polarity condition for long. It will introduce a non-linearity (which is a distortion) on the reverse polarity side of the audio signal, and continued use will either short or open the capacitor over time, depending on the failure mechanism. If you listen to music at a low volume, this may take weeks to months; if you like your music loud, the time to failure will be less.

Also, it really doesn't matter if you install the capacitor in the red wire or the black wire, and since the cap is non-polarized, the orientation in whichever wire you choose will not matter either. Some have said it might be safer in the black wire, in case the wire accidentally is grounded. Given the nature of virtually all audio amplifier designs, the black wire is not a ground or at ground, so I do not think that it is any safer to install it there. Just make sure that there is no chance that a wire can become exposed and accidentally touch the chassis of the car.

There are some people that don't like the amount of low/mid bass that this mod removes. If that is the case, try a 270uF or 330uF (or even a 400uF if you really want to hear more of the mid bass). In previous posts in this thread, the frequencies that these filter are listed, and the Amazon dot com web page to purchase these capacitors is referenced (partsexpress dot com has an excellent selection also). Remember to make sure that you get a non-polarized capacitor. Most electrolytic caps are polarized, so avoid them. BTW, as I mentioned in a previous post, the capacitors available at Radio Shack are polarized electrolytics, so do not use those. (I changed the picture I included in that previous post to convey that).

In my opinion, the 220uf capacitor high-pass filter makes the system much more accurate. Some have said that the "punch" is now gone, and I do not understand that. Punch, to me, comes from low bass, and with this mod, the deep bass that the little subs in the back produce really can now be heard. What I heard before the mod was really exaggerated mid bass which really overloaded the door mids and muddied up the midrange, which spilled over into vocals. For me, this has mod cleaned up two problems and has made the system much more enjoyable.