Ok, curiosity got the better of me and I swapped the seat belt LED (known to be working) with the airbag LED (believed to be faulty). The seat belt LED still works fine, the airbag light still doesn't light up. So it's not a simple blown LED, though I noticed I have a blown LED on my RPM cluster of 4 lights, no big deal tbh, 3 still work grand.

The mystery continues and I'm fast running out of ideas. I could use a spare MRS4 for testing purposes