The wiring on the car is from the Porsche. I think instead of purple, the wire is yellow.
It comes from the ignition switch, and connects to the solenoid on the starter. When I turn the ignition switch, I can see that it is providing 12V to the solenoid.
As shown on the diagram, there are only 2 wires to the starter - one directly from the 12V post on the battery, and the one from the ignition switch. Does that seem right?