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Unlocking the Doors or Trunk with the Key or Remote should DisArm (reset) the Alarm.
If this is Factory Alarm
You can try:Be sure Battery is fully charged.Diwsconnect the Positive Battery Cable (only).Open Driver Door & LEAVE Open.Turn Ignition Switch ON & LEAVE ON.Reconnect Positive Battery Cable.Horn may sound for a few seconds.Start Vehicle.
If none of the above work, then the Alarm Module may need to be replaced & Scan Tool may be needed to find problem with Alarm System