Have you checked the cables under the seats. They are notorious for breaking.
Might not even look broken but if you wiggle them you might feel if it is broken under the insulation.
I have repaired a couple like that. I just cut out the bad wire and soldered in a new section and then used heat shrink insulation to cover it back up.
Just an update: car went in for MOT yesterday and the airbag light didn't even get an advisory! Usual nonsense about trays fitted obscuring components, but otherwise a Pass.
No advisory for an airbag warning light being on .. that's not right !
Yeah they can't moved under trays to check items ...
You only need two tools in life WD40 and Duct Tape. If it doesn't move and should use WD40 and if it moves and shouldn't use Duct Tape.
Must admit I was surprised too, especially as it has been an advisory before.