What I did on my 21SE was that I mounted the stereo inside the console, hanging from the top. I had one of the small controls mounted on my dash. That was great for changing the volume, channels, cds, etc. On thing, you want the radio (or volume controls) easy to get to. Poly Planar makes a great unit, very waterproof, as so far, last very long. They also make a remote control for it which is great.
On my new 23SE, I mounted a poly planar up top in the t-top overhead electronics box, and got the remote control. The remote control even works through the dark glass window of the electronics box. I love that, as I can keep the electronics box closed when running, but still use the remote to control the steroe. (all my other electronics are in the console electronics box).
Speakers on the aft side of the console seem to work real well, and also stay protected.