OK. I just went out to my vehicle. The buzzer is part of the seat belt system. Try this...
get in car close door.
Turn key in ignition system (you don't have to start it just to run). Buzzer should sound. (Glow plug light will cycle)
Turn car off.
Fasten seat belt.
Turn car on to run. NO BUZZER-- but the seat belt light will still do the time out. (Glow plug light will cycle)

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92 300D 2.5L OM602 OBK #59