I'm going to try and refurb my stock ones, mostly because they have the stock wiring on them that attaches to my harness. I think it might be for a seatbelt indicator.
Looking at my photos I have to say they look like **** now that the top is on. I need to get some better lighting or something.
you are correct - that wire is for the seat belt buzzer. Let me know if you send yours out somewhere - I'm looking to get some 3-point ones restored as well.