I better jump in here because the alarm siren on an e39 body style 5 series is under the right rear fender liner. There is no alarm module under the rear seat, the alarm module is incorporated into the ZKE general module which is under/behind the glovebox. There is no option which will allow you to use the vehicles horn and trying to hook a different siren into the system may cause damage. I would cpmapre your chirp with another "like car" if possible and if yours sounds different then look into replacing your siren with a factory part.
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