Re: Installing new radio but...
I have a 1998 Bayliner Capri that had the old original cassette deck in that I just removed. Once I pulled it out all I had was the red power, black ground, antenna, and four wires to the two existing speakers which were clear. Of course the wiring harness for a new Dual marine cd/am/fm/usb/mp3 stereo was not quite the same.
Install notes: I wanted to save and use the same splash guard so I used a small router (a dremel would work too) to cut away the glastic backing so the new radio sleeve would fit. once that was done I mounted the sleeve by sliding it in and pushing back the tabs on all four sides to hold it in place.
Wiring notes: At the time I did not know I could fish a wire all the way around the bow of the boat to reach this radio which is on the passenger side dash, not within the captains console. So I wired both the power (red) and trickle power (yellow) to the single red wire, ground to ground, and two new speaker wiring to the rear left and right. I labeled everything and it is working fine. One guy I talked with said he would have ran another wire from the key as he likes to know when the boat is off, everything is off.
(note, I am a novice at this stuff and watch several youtube videos about installation etc, mostly on cars, to ramp up)
Hope this helps, SOB