Backup alert is behind the switch panel on the dash (switch panel is the panel with the rocker switches for left turn, right turn, etc.; it's in the center of the dash halfway between driver and passenger). If the tone is too loud for you, just remove the four screws holding the panel, gently pull out the panel. The buzzer is a squarish piece with a hollow. You can stuff some foam into the hollow, and that will greatly mute the sound. Once the sound is to your liking, just put the panel back in place.
I ought to do a short video and post that--really easy to do.
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