WTF? Dash won't stop beeping...

[blinker337]

OK, still a little new to this car. I drove the lady's new (to her) 87 528e to work this morning, and when I park it, the warning beeper starts beeping and won't stop. Car's in park, check. Headlights are off, check. Parking brake is up, I don't know what else it could be, and it's out in the parking lot still beeping! Even when I restarted the car and parked it again, it won't stop.

What is going on??? :help

[blinker337]

I figured out what it was. There's a plug I discovered yesterday underneath the driver's seat, and it was unplugged. Now I know why, but I don't know what the plug is for. Maybe the seatbelt sensor or something?