The m-unit does not have a built-in horn. It sends power to yours, so the horn itself needs a ground wire to chassis, and the power lead from the m-unit. The handlebar button sends a ground to the left side.
If your battery voltage gets low, the m-unit will beep-beep at you. It can also do this when you trip a circuit and it resets that circuit. Watch the m-unit while you press the horn button. The left light and corresponding right side should both illuminate solid only while the button is depressed. If the left side flashes, you're shorting it out somewhere.
The m-unit does have a built-in ALARM function though. This too uses your horn to sound.