Typical starting system w/ MT has the clutch switch and starter relay. Immobilizer/DME is independent from the starting system, from what I have seen.
I never worked on porsche but when you have problem like this, I find it best to work backward and test the starter wire and work your way back to the starter relay and back to your ignition switch. It's a pretty simple diagnostic job, IMO. Or the other way around.
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