Ford Mustang Owner's Manual page 97

Triggering the Alarm System
The armed system will be triggered if:
any door is opened without disarming the
system first (see previous paragraphs)
the trunk lock cylinder is removed
NOTE: You can open the trunk with your key
without setting off the alarm.
Disarming a Triggered System
The alarm can be disarmed by:
unlocking a front door with the key
using the UNLOCK control on the remote
entry transmitter
NOTE: The flashing lights and honking horn
will shut off automatically within 2
minutes 45 seconds after the system is
triggered. It will trigger again if
another intrusion occurs. However, the
starter circuit remains disabled until
the system is disarmed.
Coded Key Anti-Theft System
Your vehicle is equipped with the coded key
anti-theft system, you can only use an
electronically coded key to start the vehicle.
During each vehicle start sequence, the coded
key is read by the anti-theft system. If the key's
ID code matches that in the anti-theft system,
the vehicle is enabled to start. If the key's ID
code does not match the anti-theft system, or if
no coded key is detected, then the vehicle is not
able to start. Your vehicle was equipped with
two electronically coded keys. The coded
ignition key is larger than normal due to the
electronics located in the head of the key.
Additional spare keys, maximum of 14, can be
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