get a wiring diagram for this car, and trace all the starter switch wires from the key to the engine's starter components.
look closely for burnt, broken, or hacked wires and splices, and go from there.
as i'm not seeing what you see, it sounds like a complete replacement harness for the whole vehicle would be a good idea.
lacking that, you will have to trace each item's circuit from under the dash to under the hood to really see what kind of hackery has been done.
i believe you will learn a LOT about wiring with this car.