You'll probably find many threads on this in the Audio Visual subforum, including some wiring diagrams. Having just installed speakers in the 3rd row, I can confirm that you can install a new Amp and wire up speakers and the sub without disrupting the stock control panels and displays.
www.odyclub.com
I went cheap and only powered the 3rd row speakers with the new Amp, leaving the first and second row stock and it is embarrassing how underpowered the stock setup sounds now.

Audio, Video, Security & Navigation
What's the latest gadget or gizmo available for the Odyssey?
I went cheap and only powered the 3rd row speakers with the new Amp, leaving the first and second row stock and it is embarrassing how underpowered the stock setup sounds now.