Helping Us With Boats

I just bought a used 1990 sprint 266 bass boat with a 1990 Mercury 115 hp outboard motor (2-stroke). A couple times when I have turned the key to the accessory position (where the electrical system is engaged but before the engine turns over), I get a high-pitched, sustained tone. Is this a warning tone? [...]

I hooked up the black to the existing black wire, and the red to the existing red wire. I have I have nothing to connect the yellow wire on the stereo to. With the black to black and red to red, I get no power to the stereo when the ignition is in accessory position. [...]