When I first got my '89 240 wagon, I disconnected one of the little boxes
near the steering column to get rid of the annoying seat belt alarm... I
always use the belt, but it just bothered me. Unfortunately, I also lost the
"lights on" alarm at the same time, but I considered it a fair trade! As I
recall, I removed the baffle under the dash on the drivers side, then got
the seat belt alarm going so I could find out which "box" it was, and
disconnected it..
In retrospect, I think there's a connector at the seatbelt catch that can be
undone, and that might eliminate the noise, although I've never tried....