The harness that most use is one that has a male / female plug type connection so that there is no need to cut any part of the OEM harness........simply plug it in to the OEM (existing)........then use "butt" connectors to attach the wires from the new head unit to the connection wires of the harness adapter.

This leaves the ability to return the car to "Stock" if you would like to plug the OEM head unit back in.

 

I didn't cut any of the stock harness in the car, instead of using butt connectors I just soldered the head unit harness wires to the wires of the harness adapter, never liked using butt connectors and Ive always learned less air = less resistance