Taking a shortcut on this ground wire might sound like the easy answer, but I have seen two different smart sirens melt down from poor grounds and poor supply voltage to the red wire that goes to the harness. A vehicle chassis ground is a "floating" ground. The ground on the battery is a true ground. Part of the microprocessor in the smart siren is measuring the differences between these two. I don't remember why it is done that way by Federal any more, but I know that is what is going on. That is why it is important to do it correctly. Also, make sure that your red supply wire to the harness is capable of drawing 20 amps and ALWAYS a minimum of 12 volts. If it drops below this, you can fry a microprocessor or one (or both) of the transistors on the back of the box. (those two black covers on the unit). That red wire controls the keypad AND the siren! That is where the siren gets its juice, not from the main supply line into the box! Keep in mind that it may not fail right away if not wired correctly, but over time improper wiring will take its toll on these systems. Hope this helps.