I don't think it's a foregone conclusion that all the wiring in the car is going to fail. The engine wiring harnesses came from a subcontractor in Sweden. Most of the wiring in the car was installed at the factory in Germany. You don't really say how it's deteriorated but if the insulation is crumbling to dust it would be wise to do the whole length. Seems doubtful though. The color coding is very good on these things but the best place to start is with the wiring diagram for the car - which is on the service CD.