Chinese coolster atv
 

I bought 125CCfor a little of nothing and when I got it home I noticed that peer wire that holds the fuse was completely melted so I decided to get a new wiring harness for this model I hooked everything up and when I turned key on it blew the fuse so I unhooked everything but the key switch and when I turned key on it blew the fuse again has anyone ever heard of this happening before thanks

Most obvious explanation is the key switch wired wrong or shorting out internally. On most "kids Quads" a kill switch is wired through the ignition switch, and the switch has two separate circuits, one works the kill which goes to earth, one the live from fuse, which goes to starter circuit, lights & horn (if fitted). Wire it wrong and you can get the live connecting to earth.

have you checked all components that plug into wire harness for shorts? I think it would be a safe bet that something is grounding out from one of the components. A good spot to start is anything running down the bars where they can rub when turning