Wiring a head unit and in-dash LCD
I'm not sure where this belongs, so appologies if you've seen it twice.
Two questions, one about -ve and earth, and one about splicing wires:
I just bought an in-dash LCD monitor, and I need to wire it in. What I dont understand is why there's a -ve AND an earth wire? It shows in the wiring diagram a positive lead to the positive battery terminal, a negative lead to the negative battery terminal, and earth lead and an ignition lead. I thought the negative terminal of the battery was earthed to the chassis anyway? Am I missing something here?
I also want to keep my head unit as well as the in-dash LCD. So that means I have two devices that need to be connected to the ignition, accessory, earth, and power. The power is no problem, because I've run a cable directly from the battery. But what about the ignition, accessory and ground? Can I just split them and run the same lines to two devices?