How do you disconnect the wiring harness on a 98 Pontiac Sunfire?

You seriously do not want to start doing that unless you can follow the wiring diagrams in the back of your Haynes manual. Even then, it's a tough job, because even if you are successful you are going to end up with miles of greasy, filthy cables, the color of some you can only guess at, some of which are wrapped tightly in electrical tape. Don't even think about it unless you are doing a total body-off restoration of something.