89 sierra o2 sensor wiring problem

I have a 1989 gmc sierra 2500, and the Front Oxygen sensor wire either melted off or broke off. I went and bought a new sensor and cannot locate the plug to plug it into. I have contacted a gmc dealer and the only info they sent to me was of what the o2 sensor looks like, not where it plugs into. The truck is failing the emissions part of our state inspections for the NOX only and I am trying to find out where the plug is. It looks like I need to get into a harness to locate, but which one. I have spent a couple of hours lookig for the plug and it it nowhere to be found. Can anyone help?