That is the swivel seat sensor. there is a little magnetic sensor in the seat base that is only lined up when the seat is facing forward. So one of three things have happened.
1. one of the seats is facing backwards when you start up.
2. The seat is swiveled around, but not all the way.
3. The magnetic switch got knocked off when the seats where swiveled, wire got cut, switch is not lining up.
If it were my personal unit, I would tape the the switch together and take it off the seat or delete the whole thing. On the Mercedes, they simply have a warning in the owners manual that says "you must have the seats facing forward when driving". No need for a buzzer. Chrysler for some reason wants an audible buzzer as a safety precaution...but yet, no Class A's have that, nor Mercedes and they do just fine.
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