I had the same issue......even though everything looked closed and secured. I didn't NEED the buzzer, and it didn't interfere with the ignition interlock, so I just found it and cut the wire. Your rear door is likely on a separate circuit, and should have it's own buzzer.
As is always the case, after you cut any wire, immediately check and make sure the bus will start. I've read a lot of horror stories on here.....