Oil pressure light on at work time

My buddy has a '91 Escort GT and the oil pressure light won't go off and it's on at the wrong time. When the car is running the light is off and stays off, but when you turn the car off the light comes on. I have to say it's quite puzzling because it's operating opposite of the way it should. The key isn't even in the car and the light is on.

I'm thinking it's perhaps foul play with the wiring but I don't have the wiring for the light and sensor nor can I seem to find it. Does anyone have this information or problem and know how to fix it.

It does sound like a wiring issue, but the only place I know of to find a good wiring diagram, is a Haynes manual.