Injectors are switched to earth at the ECU. They will have voltage at the injector and the ECU earths the return wire to open the injector. You should expect to see 10v/ms on cranking with a scope. You could always use a LED at the injector terminal (disconnected) and see if it flash's when cranking. Try the LED in 2 ways as they are pos/neg. If no flash, the injector driver in the ECU would be suspect, because you are getting a spark at the plugs. I would tend to believe the they operate on the same input signal. If the LED flashes, the replace injectors.
Try some of the US site's on Porsche, as they can have a lot of info, but be careful of the wiring dia, as they will be slightly different the euro models.

John