Broken lower bolt from 90 seatbelt mounting

Hi (it’s my first post. Hope I’m in the right place!)
The bottom bolt sheared off as I was fitting the drivers side seatbelt mount on my 90. (Pic attached)
I can’t seem to get to it. It’s almost as though it’s within the side panel.
Any ideas??
Many thanks

Dunno but I'm guessing it's probably a rivnut. Careful drilling of the stud and an extractor looks like your next step.

As above, looks like a rivnut so drill out bolt thread with a good quality sharp drill bit. You'll need to replace the rivnut too, this is easy to drill out but be careful the drill bit isn't dragged into the parent metal. Clean area, paint rivnut hole before fitting new rivnut to stop future rust, then fit. When winding in next bolt use a good grease to ensure no galling to new rivnut and wind in slowly - do not use a driver when winding in to aluminium rivnuts.