I think in the main the oxidisation is caused by long term reaction with the plastic sheath of the wire. As Kim says it can be quite oxidised miles away from a break or join. Years back tinned stranded wires were specified.
Making a loom from scratch is not an undertaking to be taken lightly, even when you have an old one to copy. Many published wiring diagrams have errors. I'm sorting wiring on a JCB 803 digger at the moment, and there are basic errors in the diagrams that I can see have perpetuated from earlier model through to this one. An example is the dual speed windscreen wiper where the actual rocker switch to operate it isn't on the drawing !
Making a loom from scratch is not an undertaking to be taken lightly, even when you have an old one to copy. Many published wiring diagrams have errors. I'm sorting wiring on a JCB 803 digger at the moment, and there are basic errors in the diagrams that I can see have perpetuated from earlier model through to this one. An example is the dual speed windscreen wiper where the actual rocker switch to operate it isn't on the drawing !