Question on missing relay under dash
 

I've recently taken on the task of replacing my carpeting and dashboard cover in my '71 911T Targa. I had to pull the Behr A/C down in order to get the carpeting out from under the dash, and I discovered an absolute mess of aftermarket audio wiring... most of it was abandoned from a prior installation. After getting all of the janky wiring pulled out, I started looking around for any problems that might be lurking below the dash. I discovered a round relay socket mounted just to the right of the glovebox, and it was missing its relay. Does anyone know what function that relay supports? The only thing that is not working at this point is the rear window defogger, but I also see non-factory "re-wiring" present at the defogger switch, and I have not yet started to troubleshoot that.

Any ideas what that relay socket to the right of the glove box is? Thanks...

Seatbelt buzzer