I was going to do the same thing on my 04 but went with XM Sat radio and a comm system. I had a Car radio on my PC 800 with speakers in the dash. When I would get over 80 mph, it was hard to hear. With the comm system, 80 is clear as a bell. I use the FM radio on my MP3 player when I am in the big city and I want local radio.
When I was looking at the Car radio side, a Sony will JUST fit inside the left side pocket. You will have to take the pocket out of the bike to mount it. Sony makes a marine remote that will work with the radio. Others have used other brand radios. Do not get the Honda speakers. They are junk. I think David Silver in the UK sells the speaker boxes that fit the ST. Some have made it work without the boxes.