The Mercury Service Manual asks me to ground the "tan" wire of the
warning horn unit to test it. I don't understand what this means. Can
I simply run the two wires from the warning-horn unit to either
terminal of a 12-volt battery and see if I can get it to horn?



It means exactly what it says, look on the horn unit for a tan wire,
when you find it, jumper that to ground, the negative terminal of the
battery or anything connecting to that.