If the immobilizer isn't deactivated the ECU will not control the ignition coils and the fuel valves and also not run the starter. And the skreem is only for sending open/close to the bcm and send the code for the immobilizer off over CAN-Bus to the ECU.
PINOUT Skreem, you can see the pin numbers on the plug (Pinout is for Crossfire or latest SLK R170).
Pin 10 -> 12V switched
Pin 16 -> Ground
Pin 18 -> 12V continius