I have a 125CC Loncin ATV. The wiring is a complete mess. I will start the engine shut the key off and it will stay running, not even have the key in the ignition at all and the lights will work and the engine will crank and also sometimes fire up without the key.

I have a new wiring harness ready to be installed, but after I install it will it still do the same thing?

 

Could be the key switch internal contacts are staying (closed) like having the key switch on even though it's turned off, try turning the key switch on and off multiple times real fast to help clean the contacts, better yet spray with electrical contact cleaner where key goes in or anywhere where it will penetrate and flow into it.