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Hi, welcome to Just Answer. My name is***** problem is usually caused by a faulty alarm siren. There is an internal battery in the siren that will fail and cause the siren to act erratically. Replacing the siren will more than likely solve this issue. Another possibility is a faulty PSE module located in the trunk. Water can find it's way into the PSE module and cause erratic behavior or the siren. It is more likely that the siren is faulty.
Let me know if you have any questions.
The siren is located under the wiper cowling at the base of the windshield next to the wiper motor. The battery in the siren is not serviceable.