INSPECTION PROCEDURE 1: Communication with scan tool MB991958 is not possible (Communication is not possible with SRS).

CIRCUIT OPERATION
The SRS-ECU is powered from the ignition switch (IG1).
The SRS-ECU power is supplied from two circuits. Even if one circuit is shut off, the air bag can inflate.
The SRS system diagnosis can be done by connecting scan tool MB991958 to the data link connector.
TECHNICAL DESCRIPTION (COMMENT)
If communication is not possible with the SRS only, the cause is probably an open circuit in the on-board diagnostic output circuit of the SRS or in the power circuit (including ground circuit).
TROUBLESHOOTING HINTS
Damaged wiring harnesses or connectors
Malfunction of the SRS-ECU