Thank you Jimmytee for a fast response! My thinking is if there are speaker leads for rear speakers already on the bike why not just try a set of speakers. I would think that Honda is using the same radio/amp system as in a Goldwing, which powers the rear speakers just fine. If not, I guess adding an amp or amplified speakers is a must. Heck, on radio itself there is fade control, which I assume is there if rear speakers are also added. I put mine on just the front speakers to make sure they're as strong as possible.
Adding rear speakers on chopped tour pak seems to me as very clean installation, I'd love to try.