This works on all '07 Mustangs, not sure about other model years:
- Turn the ignition switch to the RUN (or ON) position. Do not start the engine.
- Wait until the safety belt warning light turns off (approximately one minute).
- At a steady, moderate speed, buckle then unbuckle the safety belt 9 times. (This step must be completed within 50 seconds of the safety belt light turning off, or else the procedure must be restarted.)
- The Airbag Light will turn on for 3 seconds. Within 7 seconds of the Airbag Light turning off, buckle then rebuckle the safety belt one time.
- The Airbag Light will flash for 3 seconds to confirm a setting change to disabled, or will flash twice for 3 seconds each to confirm a setting change to enabled.