What am I missing? (Wiring question)

[DissFigured]

Logically, something isn't right.
I bought a wiring harness and for example, there will be one wire labeled "horn" however, the horn has 2 wires coming from it in order to complete the circuit. Same w/ the tail ligkts. I see a tail light with 2 wires coming from it, the pos and neg however, the wiring harness has a single wire called left rear tail light.

What piece of information am I missing here?

 

[gbarber]

My horns only have one wire (pos.) going to them. The neg. wire isn't needed because they ground directly to the radiator support.

 

[LMan]

I'm guessing that the harness has a common ground for that circuit, instead of each item grounding individually.

But that's a SWAG.