Quote Originally Posted by daug1502 View Post
Not very common to switch on the ground side of things. However I was taught to wire houses and cars tend to be backwards since they are Negative ground systems. Houses are Hot on the Negative side, Neutral on the Positive side, and if it's newer that third prong on there is your ground.
You are comparing apples and oranges. Houses are alternating current and cars are direct current. In AC electron flow is in both directions, in DC electron flow is actually negative to positive. It is common to see the negative open. All of these devices are controlled through relays, so there is no direct horn to switch connection, switch activates a relay that supplies current.